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.
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.