I recently switched from Plex's pretty terrible native players to Plezy (on iOS, iPadOS and tvOS).

Plezy supports both Plex and Jellyfin as a backend - I'm not sure if there is 100% feature parity.

Not sure if there is for Apple TV

but they have an iOS app for ipad/iphone https://apps.apple.com/us/app/jellyfin-mobile/id1480192618