How to receive callbacks by using Rx.NET?

Question

Suppose I have 3 student ids.

Things I want to do:

1. Fetch 3 students information asynchronously.

2. If callback not comes within 4 seconds it will retry for three times for that network call.

   Here is my code:

   Dictionary<string, SemaphoreSlim> semaphoreSlims = new Dictionary<string, 
   SemaphoreSlim>  
   {
       { "1", new SemaphoreSlim(0, 1) },
       { "2", new SemaphoreSlim(0, 1) },
       { "3", new SemaphoreSlim(0, 1) }
   };
   
   IObservable<string> studentIds = new string[] {"1", "2", "3"}.ToObservable();
   
   private IObservable<Student> LoadStudent(string id) {
       return Observable.Create<Student>(
           async o =>
           {
               StudentFoundDelegate studentDetailHandler = (Student student) =>
               {
                    // callback found
                    semaphoreSlims[id].Release();
                    o.OnNext(student);
               };
   
               StudentNotFoundDelegate errorHandler = (string representation, Result eCode) =>
               {
                    // callback found
                   semaphoreSlims[id].Release();
                   o.OnError(new Exception("Student Not Found"));
               };
   
               // Api calls
               StudentManager.Instance.GetStudentDetail(id, studentDetailHandler, errorHandler);
   
               await semaphoreSlims[id].WaitAsync();
   
               return Disposable.Empty;
           });
    }
   
    Observable.Timeout(studentIds.SelectMany(id => LoadStudent(id)), TimeSpan.FromMilliseconds(4000))
            .SubscribeOn(ThreadPoolScheduler.Default)
            .Retry(3)
            .Subscribe(
               student =>
               {
                   Logger.LogMessage("Found Student "+ student);
               },
   
               (ex) =>
               {
                   Logger.LogMessage(ex + "");
               }
    );
   

But, i am not getting any callbacks. Can anyone suggest where am i wrong and what is the proper way to handle it. Thanks.