If you are seeing your routers config page, and you are sure you are connecting from outside your network, it sounds like the router's 443 page is overriding the port forwarding. Otherwise, it's like @fixmycode@feddit.cl said and you just need a local DNS that points to the right spot locally, and let your public DNS point for external connections.
As for the hosts file, you can see a guide here for windows/linux/mac. Basically this is a override of any DNS entries. Here you can point jellyfin.domain.com to your jellyfin servers LAN IP and test the connection works.