Play mp4 with subtitle on iOS device
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