that surely is the issue. you can convert it to mp4 with ffmpeg:
ffmpeg -i input.mkv -c copy output.mp4
If you want to keep subtitles this will probably work:
ffmpeg -i input.mkv -map 0 -c copy -c:s mov_text output.mp4
orgmode with neovim on PC and orgzly on phone. syncing with syncthing
Edit: I'm actually using orgzly revived, a community maintained version of orgzly, since orgzly is no longer mantained
For anyone who is interested in note taking in your everyday editor like vim or emacs, orgmode is an emacs tool (in neovim there is a clone plugin) for note taking, todos, agendas, etc. It uses a format similar to markdown, and a good part is that with the orgzly app you receive notifications for your events. So basically you can use orgmode as a calendar as well (I do!).
Not even a raspberry pi with lineage os?
I know. As I said, I'd be using Jellyfin for the most part.