Unity Async or Coroutine WebRequest and wait on data

The Idea is, that my Data get loaded, asynchron, when my app startet. Because the user normaly need some seconds to interact and in this time, i should get my data. (For understanding reasons, it will be a book app, with some books)

To do this, in loadingprofile i open a method on a static class to start loading. In the Screen where i want to show my data, i check with a method on the static class, if the data are there ( and wait if theire not).

So in the Profil loading screen i did this:

public class LoadProfiles : MonoBehaviour
{
    private void Awake()
    {
        if (!LoadingData.dataLoaded)
        {
            LoadingData.LoadData();
        }
    }
    void Start()
    {
        //Loading Profile Dataa
    }
}

Then when the User click on a profile, i open the next scene "Bookshelf" and a game object in it has the following script attachted to read the data (the book categories). so i open the wait on data method, that shoud return, when the data are arrived.

public class PopulateShelf : MonoBehaviour
{
    private void Start()
    {
        LoadingData.WaitOnData();
        LoadingData.Categories.ForEach(c => {
            //create categories
        });
    }
}

The static loading class looks like the following

public class LoadingData : MonoBehaviour
{
    public static List<CategoryDTO> Categories;
    public static IEnumerator CategoriesRequest;
    public static bool dataLoaded = false;

    public static void LoadData()
    {
        CategoriesRequest = LoadCategories();
        dataLoaded = true;
    }
    public static void WaitOnData()
    {
        if (!dataLoaded) LoadData(); 
        while (CategoriesRequest.MoveNext())
        {
            //Log wait on categories
        }
    }
    public static IEnumerator LoadCategories()
    {
        UnityWebRequest request = UnityWebRequest.Get(Constants.apiURL + Constants.apiURLCategories);
        yield return request.SendWebRequest();

        while (!request.isDone)
        {
            yield return true;
        }

        if (request.result == UnityWebRequest.Result.ConnectionError)
        {
            //Log error
        }
        else
        {
            Categories = JsonConvert.DeserializeObject<List<CategoryDTO>>(request.downloadHandler.text);
            Categories.Sort((x, y) => x.Order.CompareTo(y.Order));
        }
    }
}

So, the problem is that the on the loging screen, it opens the LoadingData.LoadData Method (but it do not start loading).

When im in the Bookshelf Scene, and open LoadingData.WaitOnData it starts loading the data and the User have to wait.

I also tried it with StartCoroutines. I refactored the LoadingData class, to a non static class. Startet the Coroutings in the LoadData Method.

First it looked like it worked, the data was loaded in the profileScene

Because of it was not static, i had to pass somehow the Object. To do this i created in a static script a Constants for a static LoadingData script variable.

In the Profile Scene i attached it and loaded the data. When i wanted to read it in the BookShefScene, i got a null refrence in my static class... Here would be the class with coroutines

public class LoadingData : MonoBehaviour
{
    public Dictionary<int, BookDTO> Books = new Dictionary<int, BookDTO>();
    public List<CategoryDTO> Categories;
    public bool dataLoaded = false;

    public void LoadData()
    {
        StartCoroutine(LoadCategories());
        StartCoroutine(LoadBooks());
        dataLoaded = true;
    }
    public void WaitOnData()
    {
        if (!dataLoaded) LoadData();
        while (categorieLoaded)
        {
           //log waiting
        }
    }
    public IEnumerator LoadCategories()
    {
        UnityWebRequest request = UnityWebRequest.Get(Constants.apiURL + Constants.apiURLCategories);
        yield return request.SendWebRequest();

        while (!request.isDone)
        {
            yield return true;
        }

        if (request.result == UnityWebRequest.Result.ConnectionError)
        {
            //Log Error
            
        }
        else
        {
            Categories = JsonConvert.DeserializeObject<List<CategoryDTO>>(request.downloadHandler.text);
            Categories.Sort((x, y) => x.Order.CompareTo(y.Order));
        }
    }
}

in profile loading screen, to attache my script to the static variable, i said something like this:

Constants.loadingData = this.GetComponent<LoadingData>();
Constants.loadingData.LoadData();

and in the bookshelf i said

Constants.loadingData.WaitOnData();
Constants.loadingData.Categories();

I checked with the Debuger, in the Profile Scene the Constants.loadingData was created and referenced. In the Bookshelf Scene, i dont saw the referenced object (it was null)...

can anyoune help me with one of this solution? or have a third solution?