Adding an error to ModelState and redirecting to the same page not working in ASP.NET MVC

I have a website on a local server where users can send me a message. I capture the name, email and the message from the user from a modal on my form.
Now I have a controller that checks if that email already exists and gets the last time that email address posted a message to the server and computes if the interval is less than 30 minutes then adds a Model error to the Index Page model and redirects to it.

In the model of the Index Page, I check if the model state is valid and display the error you are only allowed to send a message every 30 minutes and display that error inside a span tag but the ModelState.IsValid() is returning true in my case, why is this when I have already modified it. Am expecting the user to be shown the message in red after user attempts to open the modal on redirect.
The controller method that handles the request is this

public IActionResult GetAction([FromForm]string Name, [FromForm] string Email, [FromForm] string Query)
        {
            //make sure the same user allows 30 minutes to elapse before sending a new message
            //check if that email has already sent an sms
            if(_dbCtx.ClientMessages.ToList().Any(x=> x.Email == Email)){
                //this means the user posted a message now, get the last time he posted
                var date = _dbCtx.ClientMessages.FirstOrDefault(x=> x.Email == Email).Time;
                var minELAPSED = TimeSpan.FromMinutes(DateTime.Now.Subtract(date).TotalMinutes);
                if (minELAPSED.TotalMinutes < 30)
                {
                    //add this error to the index page model
                    ModelState.AddModelError("", "You are only allowed to send a new message once every 30 minutes");
                    //redirect to index 
                    return RedirectToPage("/Index");
                }
            }

and then inside the index.cshtml,I check for the state like this.

   <div class="modal-header">
                <h4 class="modal-title" id="myModalLabel">Contact Us</h4>
                @if (!ViewData.ModelState.IsValid)
                {
                    //display the error message in the span
                    <span id="error-message" class="text-danger" >@Html.ValidationSummary(false)</span>
                }
                <button type="button" class="close" onclick="closeModal()" aria-hidden="true">×</button>
            </div>

Because you are redirecting, the ModelState is lost. Can you not use TempData?

public IActionResult GetAction([FromForm]string Name, [FromForm] string Email, [FromForm] string Query)
{
    if(_dbCtx.ClientMessages.ToList().Any(x=> x.Email == Email)){
        var date = _dbCtx.ClientMessages.FirstOrDefault(x=> x.Email == Email).Time;
        var minELAPSED = TimeSpan.FromMinutes(DateTime.Now.Subtract(date).TotalMinutes);
        if (minELAPSED.TotalMinutes < 30)
        {
            TempData["ErrorMessage"] = "You are only allowed to send a new message once every 30 minutes";
            return RedirectToPage("/Index");
        }
    }
    // return something else 
}

<div class="modal-header">
    <h4 class="modal-title" id="myModalLabel">Contact Us</h4>
    @if (TempData["ErrorMessage"] != null)
    {
        <span id="error-message" class="text-danger">@TempData["ErrorMessage"]</span>
    }
    <button type="button" class="close" onclick="closeModal()" aria-hidden="true">×</button>
</div>