fennec

Jellyfin is avaliable on almost everything as far as I am aware (excluding games consoles). Havent used Plex much but I know for Jellyfin the majority of my users issues relate to iOS. Id check your encoding of the specific file they have issue with M series macs can be weird with certain encodes of h.264 for example or a specific audio type. Here is a decent breakdown of codec support by client/browser https://jellyfin.org/docs/general/clients/codec-support/

Whats your setup? I just Ngnix Proxy Manager, Jellyfin etc in Docker. Modify ufw rules and also install this on the server (linux) https://github.com/friendly-bits/geoip-shell