This old question has been mine too and now I decided to convert all my iphone .mov and .mp4 files. So I just installed ffmpeg and all its dependencies and the nextcloud video converter app on my debian stretch nextcloud and NAS server.
Just did that and abrakadabra - all videos are playing just fine without converting. Simple solution. I just never installed ffmpeg because it depends on X11-common, what I never wanted on a server.