I've just tried playing one of my videos on an iPad using JRemote. My default subtitle track (flagged with a d in the metadata) was displayed without any problem. However, I've not yet found a way to select other subtitles or to turn them off in this local playback situation.
In contrast, JRemote2 on my Android phone has obvious subtitle controls for local playback, but it looks like JRemote2 is not available for iOS, so you may have to set the default flag for your subtitles.
EDIT: My reference to a d flag for indicating the default subtitle track relates to a facility in MakeMKV (and MKVToolNix). I've failed to find a way to set that flag in MC, but you can achieve a similar result by using the following settings (which take precedence over any default flag).
Options > Video > Subtitles & Language > Subtitle mode: Always use subtitles
Options > Video > Subtitles & Language > Default subtitle language: (your choice)