Try the suggestions here or [HowTo] Use Plex with No Internet to make it accessible “offline” or without " Plex Authentication".
But (for me anyway) I find it a pain in the a** to have to keep flipping things around, which is why I suggest having Jellyfin running on the same server to not have to deal with changing around Plex settings. I know it sucks not top have the “synced” watch and such, but it does work well for me with Tailscale & Jellyfin (and my Kavita Comic Book Server) for use outside of my home network.
