Problems playing video on iOS with Handheld.PlayFullScreenMovie

I have the video in my StreamingAssets folder, it is an .mp4 which unity can read but it still will not play.

 function PlayMovie () {
 
   
   print("Movie has been played");
   
   Handheld.PlayFullScreenMovie ("Movie13.mp4", Color.black, FullScreenMovieControlMode.Full);
 
 
 }

The iOS controls appear as expected but the movie does not play. I have a suspicion it is because the video is in the wrong codec, but for the life of me I can't figure out how to get a video in the RIGHT codec. I have tried half a dozen video converters, and none of them seem to encode the video correctly.

If my problem is the codec, how do I import a video in a way "Handheld.PlayFullScreenMovie" can use? What programs should I use to get the video properly prepared for Unity?