Sessions not persisting across the code in Laravel 7

I'm buiding an e-commerce website in Laravel 7 and I'm trying to do a shopping cart with sessions. I know there are many packages I can use for, and I tried to use some of them with no success, but that's not the problem I wanna talk about here.

At first, the code is very simple, I have two methods: index and add.

index method

public function index(){
    
    var_dump(session()->get('testCart'));
    die();

    return view("cart.index");
}

add method

 public function add(){

    $cart = [
        "id" => 1,
        "product" => "PS4",
        "qty" => 1,
        "totalPrice" => 300
    ];

    session(["testCart" => array($cart)]);

    return view('cart.index');

 }

That's the code, as I said before, very simple for this test. I also have a route to add products to my cart.

routes/web.php

// CART
Route::get('/carrito', 'CartController@index')->name("cart.index");
Route::get('/carrito/add', 'CartController@add')->name("cart.add");

The first step I do, of course, is go to /carrito/add and then to the route /carrito. But when I go to this last route, the code doesn't find my cart session and it gives me a NULL.

NOTE: I have noticed that sometimes the index method finds my session and shows its value. But most of the time it doesn't.

I hope my English was clear enough to describe my problem properly, if you need more code or information, make me know it.

Thanks in advance, Ciro.