Best way to delay the GameOver screen?

When the player crashes, I want to display the game over screen. Everything works fine in the original version, but now I want to delay the appearance of the Retry/Exit buttons for about 3 seconds. I also have the need to insert a call to show an interstitial ad between when the retry/exit buttons appear and the game ends. This is what I thought would work, but the buttons do not enable. I'm not worried about the advertisement call working yet, as I cannot seem to get WaitForSeconds to do anything yet on the second call.

Is there a better way to do this? If I use StartCoroutine in ShowGameOverScreen(), it doesn't get called. Also, if I use interactable on the Buttons, the text is still visible, does that require a second property change for just the text?

 public IEnumerator  ShowGameOverScreen ()
     {
         gameOverScreen.SetActive (true);
     
         yield return new WaitForSeconds (1.0f);
 
         if (adFreeVersion)
             AdmobVNTIS_Interstitial._showInterstitialImmediately ();
 
         yield return new WaitForSeconds (3.0f);
 
         buttonExit.interactable = true;
         buttonRetry.interactable = true;
 
     }