Hello!
Are you trying to view the video inside the text messaging application? I have noticed a myriad of issues when doing this after receiving a video message from an iPhone user. Have you tried saving the video to local storage > navigating to a file explorer (there are tons on the Google Play Store) > opening the video with a video player app (try a non stock one like VLC). I know this is a bit cumbersome, but its worth the test. I had the same issue with pictures received via MMS from iPhones.