I added a subtitle to a mp4 file use ffmpeg, I can turn on/off the subtitle with quickplayer on MacOS. However, the native player on iOS Photos app no longer have subtitle control/support for mp4 video file.
I try to use VLC but found VLC even does not have access to Camera Roll on iOS device.
https://forum.videolan.org/viewtopic.php?f=7&t=148089&p=486194&hilit=ios#p486194
Then I try to use PlayerXtreme to do the same thing, although it claims to have subtitle support in a paid upgrade version, but it still cannot load the subtitle correctly. I upload the video file in case anyone want to test.
Best Answer
Try to put the file into the Files app, either by exporting it from the Photos app via share menu > Save to Files or by putting it into the iCloud Drive folder on the Mac.
Them, in the Files app, select the movie file, choose the share menu here again and select "Open in VLC" (somewhere in first row of icons below the AirDrop area).
Btw. the excellent Subler macOS app allows modifying the track metadata inside the video, so the subtitle track could be shown as "English" instead of generic info.
https://subler.org