Hi there,
Sorry for the trouble. Yes, one of the mobile video clips could be causing the export to stop. Keep in mind that MP4 is only the container; the video inside may use different codecs or recording formats.
If the export stops at approximately the same percentage each time, check the clips, transitions, or effects around that point in your timeline. As a test, you can also convert the suspected mobile clip to H.264 using HandBrake or Shutter Encoder, replace it in the project, and try exporting again.
Premiere Rush is now at end of life and won't be receiving additional codec or compatibility updates.
More information:
https://helpx.adobe.com/premiere-rush/desktop/kb/end-of-life.html
Thanks,
Kevin