Problems with mp4 files

My WD TV unit seems to have issue playing mp4 files. Some play fine. Other will start and then sort of slow down or stop. Others won’t start at all and just bouce  back to the file select screen. Is there a way to fix this?

is remux the same as converting to another format?

Remux can mean putting the file into the same container format (mp4 > mp4) or another container (mp4 > mkv). In any case the quality is not touched and the file is not recoded which is why it’s usually quite fast.