This is how I started.
I have a dynamic IP and a router provided by my ISP. IP assignations, DHCP, are managed by the router. I went with DuckDNS for a free DNS service. Select a name and you get a myname.duckdns.org that you need to assign to your dynamic IP. duckdns has instructions to create a cronjob to update your dynamic IP on duckdns.org. (Routers come in all shapes and configs, chances are that this won’t work for most people) On the router, I assigned a static IP to the server hosting Jellyfin, in case of a reboot Jellyfin would always have the same IP. On the Ports page I opened up the default port for Jellyfin at that IP. I could then access Jellyfin outside of my local network using myname.duckdns.org:1234
This is not what I have right now, but it helped my get started.