VOBSUB subtitles and the mp4 format have a history not to mix well.
Step 1:
drag the mp4 file into MKVtoolnixGUI and place the resulting MKV file on your server.
If that doesn’t help, proceed to
Step 2: convert the VOBSUB subtitle to SRT, using e.g. Introduction: convert image-based subtitles to SRT with Subtitle Edit
then place it beside the video file, named as explained in https://support.plex.tv/articles/200471133-adding-local-subtitles-to-your-media/
