Blazor Server-Side variable doesn't update after function call because of OnAfterRenderAsync

I have have a Server-Side Blazor app with a simple index.razor Page. My goal is to read sessionStorage and use it's content. But for now I am trying to make baby steps to understand what is going on.

This is the html

@inject Blazored.SessionStorage.ISessionStorageService sessionStorage

<button @onclick="TestInc">TEST</button>
<h1>@tutu</h1>

I only have a text to show the the number and a simple button to increment my value And this is the code

@code {
    private int tutu = 0;
    public string sNom;

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            bool isConnected = await sessionStorage.GetItemAsync<bool>("isConnected");
            //if (!isConnected)
            //{
            //    NavManager.NavigateTo("/Login", forceLoad: true);
            //}
            sNom = sessionStorage.GetItemAsync<string>("Nom").Result;
        }
    }
    private void TestInc()
    {
        tutu += 1;
    }
}

For some reason, the code works fine if I comment the whole OnAfterRenderAsync function. The page will show the counter and update it when it I click on the button. However if I have the function OnAfterRenderAsync uncommented and do it's thing, the counter is not updated at all and if I put a breakpoint inside TestInc it never reaches it.

I am relatively new to Blazor so I might be missing something really obvious here.

What I really would like to achieve is get sessionStorage variable (a username for example) and display it inside my HTML without having to click on a button or reloading the page