Play mp4 with subtitle on iOS device

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. https://www.dropbox.com/s/uzdf10m4xqw7bmr/outfile.mp4?dl=0