Unfortunately none of these methods worked for me as I was already using VLC.
I tried downloading a different YT video, (a short song) which did not work as mp4, but worked as "WEBM Video format" using 'Video Download helper' firefox ad on.
I tried downloading the original youtube video again, Tom Woods, from a basic online site "https://y2mate.guru/en8/
" and the original video worked fine as mp4.
So not sure what's going on there. It seems to work with some videos, but not others. Maybe it was the format it was uploaded to in youtube?