When I connect my hard drive via usb to media player, it plays all files. But when I connect my hdd to the wireless router then connect media player via wifi, it registers the video files as audio files.
Common files affected are MP4 and Mpeg_4.
I can connect with my Samsung tablet via wifi and play all files perfectly.
Sounds like the issue is with your router mis-registering the files as audio instead of video files. That’s up to the DLNA server in your router to figure out.
My only suggestion would be to see if they work if you change the extension from MP4 to M4V.