Invalid Registration FCM in API using Entity framework

Question

I am trying to get DeviceToken of device in Xamarin iOS through RegisteredForRemoteNotifications and save it to Preferences, through API I save DeviceToken to database Server.

AppDelegate.cs

public override void RegisteredForRemoteNotifications(UIApplication application, NSData deviceToken)
{
    byte[] bytes = deviceToken.ToArray<byte>();
    string[] hexArray = bytes.Select(b => b.ToString("x2")).ToArray();
    DeviceToken = string.Join(string.Empty, hexArray);

    Preferences.Set("TokenDevice", DeviceToken);
}

I have saved the DeviceToken to the database, the DeviceToken of the device I get:

6b60feecad920471ccde5a3447ab22d3f820abae821daeac726cc7e6d0863465

I wrote an API to send notifications to that device:

[HttpPost]
public void SendNotification(string devicetoken, string title, string body, string link, string icon)
{
    try
    {
        dynamic data = new
        {
            to = devicetoken, // Uncoment this if you want to test for single device
                              // registration_ids = singlebatch, // this is for multiple user 
            priority = "high",
            notification = new
            {
                title = title,     // Notification title
                body = body,    // Notification body data
                link = link,       // When click on notification user redirect to this link
                icon = icon
            }
        };

        //var serializer = new JavaScriptSerializer();
        var json = JsonConvert.SerializeObject(data);
        Byte[] byteArray = System.Text.Encoding.UTF8.GetBytes(json);

        string SERVER_API_KEY = "AAAAqOId6Is:APA91bHJ5pQgLlanU8gwQwnpxdBlKS00i1xxxxxxxxxxxxxxxxxxxxxxxxxxx";
        string SENDER_ID = "72xxxxxxxx";

        WebRequest tRequest;
        tRequest = WebRequest.Create("https://fcm.googleapis.com/fcm/send");
        tRequest.Method = "post";
        tRequest.ContentType = "application/json";
        tRequest.Headers.Add(string.Format("Authorization: key={0}", SERVER_API_KEY));

        tRequest.Headers.Add(string.Format("Sender: id={0}", SENDER_ID));

        tRequest.ContentLength = byteArray.Length;
        Stream dataStream = tRequest.GetRequestStream();
        dataStream.Write(byteArray, 0, byteArray.Length);
        dataStream.Close();

        WebResponse tResponse = tRequest.GetResponse();

        dataStream = tResponse.GetResponseStream();

        StreamReader tReader = new StreamReader(dataStream);

        String sResponseFromServer = tReader.ReadToEnd();

        tReader.Close();
        dataStream.Close();
        tResponse.Close();
    }
    catch (Exception)
    {
        throw;
    }
}

Where: SERVER_API_KEY and SENDER_ID I got from in console.firebase.google.com/Project settings/Cloud Messaging

I tried to test the API that sends notifications to the device I get the error: {\"multicast_id\":7343900550378569449,\"success\":0,\"failure\":1,\"canonical_ids\":0, \"results\":[{\"error\":\"InvalidRegistration\"}]}

I referenced this article. However I want to use this code because it allows me to set the Notification link and icon. Is this an outdated notification code? I searched the forums, and didn't get any reasonable results. Looking forward to everyone's help.