How to fix UIPageViewController manual pagination?

In your code, currentIndex is getting wrong. You can use the following delegate method to get correct currentIndex.

func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool)

Also in your scheduled timer configurePageViewController methods call every 5-sec. This method creates a new instance of UIPageViewController at the time of method call and adds a child view, this will cause memory issues in the future.

I would suggest if you add UIPageViewController embedded in ContainerView using a storyboard. You can find attached Screenshot.

I have a better solution for your scenario. Please find the following code:

My ViewController.Swift code:

class ViewController: UIViewController {
    
    @IBOutlet weak var contentView: UIView!
    @IBOutlet weak var pageControl: UIPageControl!
    let myDataSource = ["Img1", "Img2", "Img3", "Img4"]
    let urlDataSource = ["https://fabric.io/", "https://about.gitlab.com/", "https://developer.apple.com/", "https://fast.com/"]
    var timer = Timer()
    var pageViewController: CustomPageViewController? {
        didSet {
            pageViewController?.objViewController = self
        }
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        configurePageControl()
        timerStart()
    }
    
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if let pageViewController = segue.destination as? CustomPageViewController {
            self.pageViewController = pageViewController
        }
    }
    
    func pageViewIdexChange(_ index : Int) {
        pageControl.currentPage = index
    }
    
    func timerStart() {
        timer.invalidate()
        guard let pageViewController = pageViewController else { return }
        timer = Timer.scheduledTimer(timeInterval: 5.0, target: pageViewController, selector: #selector(pageViewController.scrollToNextViewController), userInfo: nil, repeats: true)
    }

    func configurePageControl() {
        pageControl.numberOfPages = myDataSource.count
        pageControl.currentPageIndicatorTintColor = .lightGray
        pageControl.pageIndicatorTintColor = .white
    }
    
}

My DataViewController.Swift code:

class DataViewController: UIViewController {
    
    @IBOutlet weak var btn: UIButton!
    var displayImgName :String?
    var redirectURL :String?
    var index: Int?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        btn.setImage(UIImage.init(named: displayImgName ?? ""), for: .normal)
        view.backgroundColor = .random()
    }
    
    @IBAction func onClickBtn(_ sender: UIButton) {
        
        if let url = URL(string: redirectURL ?? ""), UIApplication.shared.canOpenURL(url) {
            if #available(iOS 10.0, *) {
                UIApplication.shared.open(url, options: [:], completionHandler: nil)
            } else {
                UIApplication.shared.openURL(url)
            }
        }
    }
    
}

My CustomPageViewController.Swift code:

import UIKit

class CustomPageViewController: UIPageViewController {
    
    weak var objViewController: ViewController?
    var myDataSource: [String] = []
    var urlDataSource: [String] = []
    var currentIndex = 0
    
    private(set) lazy var orderedViewControllers: [UIViewController] = {
        // The view controllers will be shown in this order
        var vcs : [DataViewController] = []
        for i in 0 ..< myDataSource.count {
            vcs.append(detailVCAt(index: i)!)
        }
        return vcs
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        dataSource = self
        delegate = self
        
        myDataSource = objViewController?.myDataSource ?? []
        urlDataSource = objViewController?.urlDataSource ?? []
        
        if let initialViewController = orderedViewControllers.first {
            scrollToViewController(initialViewController)
        }
    }
    
    func detailVCAt(index: Int) -> DataViewController? {
        
        if index >= myDataSource.count || myDataSource.count == 0 {
            return nil
        }
        
        guard let dataVC = storyboard?.instantiateViewController(identifier: "DataViewController") as? DataViewController else {
            return nil
        }
        
        dataVC.index = index
        dataVC.displayImgName = myDataSource[index]
        dataVC.redirectURL = urlDataSource[index]
        
        return dataVC
    }
    
    //MARK: - Methods
    @objc func scrollToNextViewController() {
        if currentIndex < orderedViewControllers.count - 1 {
            scrollToViewController(currentIndex + 1)
        } else {
            scrollToViewController(0, direction: .forward)
        }
    }
    
    func scrollToViewController(_ newIndex: Int, direction: UIPageViewController.NavigationDirection? = nil) {
        var direction = direction
        if let firstViewController = viewControllers?.first,
           let currentIndex = orderedViewControllers.firstIndex(of: firstViewController) {
            if direction == nil {
                direction = newIndex >= currentIndex ? .forward : .reverse
            }
            let nextViewController = orderedViewControllers[newIndex]
            scrollToViewController(nextViewController, direction: direction!)
        }
    }
    
    func scrollToViewController(_ viewController: UIViewController,
                                direction: UIPageViewController.NavigationDirection = .forward) {
        setViewControllers([viewController],
                           direction: direction,
                           animated: true,
                           completion: { (finished) -> Void in
                            // Setting the view controller programmatically does not fire
                            // any delegate methods, so we have to manually notify the
                            // 'tutorialDelegate' of the new index.
                            self.notifyTutorialDelegateOfNewIndex()
                           })
    }
    
    private func notifyTutorialDelegateOfNewIndex() {
        
        if let firstViewController = viewControllers?.first,
           let index = orderedViewControllers.firstIndex(of: firstViewController) {
            currentIndex = index
            objViewController?.pageViewIdexChange(index)
        }
    }
    
    
}

// MARK: - UIPageViewControllerDataSource & UIPageViewControllerDelegate
extension CustomPageViewController : UIPageViewControllerDataSource , UIPageViewControllerDelegate {
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        guard let viewControllerIndex = orderedViewControllers.firstIndex(of: viewController) else {
            return nil
        }
        
        let previousIndex = viewControllerIndex - 1
        
        // User is on the first view controller and swiped left to loop to
        // the last view controller.
        guard previousIndex >= 0 else {
            return orderedViewControllers.last
        }
        
        guard orderedViewControllers.count > previousIndex else {
            return nil
        }
        
        return orderedViewControllers[previousIndex]
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        guard let viewControllerIndex = orderedViewControllers.firstIndex(of: viewController)  else {
            return nil
        }
        
        let nextIndex = viewControllerIndex + 1
        let orderedViewControllersCount = orderedViewControllers.count
        
        // User is on the last view controller and swiped right to loop to
        // the first view controller.
        guard orderedViewControllersCount != nextIndex else {
            return orderedViewControllers.first
        }
        
        guard orderedViewControllersCount > nextIndex else {
            return nil
        }
        
        return orderedViewControllers[nextIndex]
    }
    func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
        
        notifyTutorialDelegateOfNewIndex()
    }
    
}

My StoryBoard image

The issue in your code is that you cannot determine the index of the currently visible view controller, inside pageViewController(_:viewControllerBefore:) and pageViewController(_:viewControllerAfter:) functions of the UIPageViewControllerDataSource, due to the fact that those delegate functions may get called more than once each time you scroll.

For example, if you are currently in index 0 and you scroll to index 1, the pageViewController(_:viewControllerAfter:) function will be called 2 times, one for index 1 and one for index 2 to preload the next page.

Having said that, you can pretty much find out the index of the currently visible view controller following this answer and using pageViewController(_:didFinishAnimating:previousViewControllers:transitionCompleted:) function of the UIPageViewControllerDelegate, but with a small modification that fits your needs:

extension ViewController: UIPageViewControllerDelegate {
    func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
        guard
            completed,
            let currentIndex = (viewControllers?.first as? DataViewController)?.index
        else {
            return
        }
        currentVCIndex = currentIndex
        pageControl.currentPage = currentVCIndex
    }
}

Also, there is no need to implement presentationIndex(for:) and presentationCount(for:) functions of UIPageViewControllerDataSource since you are using a custom UIPageControl. So, your extension may look like this:

extension ViewController: UIPageViewControllerDataSource {
    func pageViewController(_: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        
        let dataVc = viewController as? DataViewController
        
        guard var currentIndex = dataVc?.index else {
            return nil
        }
        
        if currentIndex == 0 {
            return nil
        }

        currentIndex -= 1
        
        return detailVCAt(index: currentIndex)
    }

    func pageViewController(_: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        
        let dataVc = viewController as? DataViewController
        
        guard var currentIndex = dataVc?.index else {
            return nil
        }

        if currentIndex == myDataSource.count {
            return nil
        }
        
        currentIndex += 1
        
        return detailVCAt(index: currentIndex)
    }
}