Unable to play MPEG-4 videos

Maybe there is some incompatible codec.

Apps like IINA can play almost any codec.

https://iina.io/

Or post a sample clip to some cloud service and post a link there (not Youtube because it re-encodes movies).

Or check the movie specs via MediaInfo > View > Text and copy&paste it here via the "Additional text" icon in the bottom of the reply window here.

Get MediaInfo v26.01 from Mac App Store!