Uhm Jellyfin Media Player on a computer ? if yes, you can use KDE Connect on your phone and computer. it's an awesome tool!
Jellyfin: The Free Software Media System
Current stable release: 10.10.0
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Yatse does that. It has more bells and whistles if you pay for premium or whatever it's called, but if you don't venture outside the first screens, it can be customised into a great very basic remote.
Can Yatse be used to control Jellyfin Media Player?
I only use it to control my Kodi clients with the Jellyfin plugin but it does that wonderfully.
I was wrong, I'm afraid. It turns out that yatse can access the jellyfin server, but jellyfin needs a player, not a remote control. Yatse can do that, but it's not a remote control. Sorry!