Can't get .mp4 file onto iphone's photo app off creative cloud

I have a portrait video created for instagram, in Premier Pro.

Using H.264. Preset Match Source - High Bitrate.

1080x1920

I've never had a problem before with this. Only difference is the portrait orientation.

When I try to copy from windows to iphone via lighning cable I get an error message and the transfer doesn't start.

When I try to download it off CC I get the usual Preparing Video dialogue. When it gets to 100%, I get a Media Type Not Supported message.

Tried using https://uploadfiles.io/ & managed to get the file onto the phone's internal storage. Does not appear in photos. Cannot be opened by instagram or any othe app for that matter.

Is there an obvios reason that this happening? Is it a premier export setting issue?

Cheers.