What you are looking for is a VPN or to port forward Jellyfin. Not to be confused with a Commercial VPN. You want him to Virtually connect to your Private Network. Personally, I don't trust port forwarding Jellyfin directly. So, I setup a wireguard VPN and port forwarded that. Then, connect remotely through the VPN to the local Jellyfin. Idk about Roku though. I cannot say wether they have a good wireguard app.

I’m sure you could, biggest issues are exposing yourself to security threats. Most home networks are pretty secure due to the fact that in general using any router locks down your network pretty hard. So to access jellyfin remotely you would need to poke holes in your security.

Second issue is network bandwidth/throughput limitations. But if you are happy with your speeds then it’s just a matter of keeping your server up and running, especially if you are away from home for any length of time.

Personally I would really suggest a seed box of some description. It will really change your piracy life. I could never go back. And typically you can run plex/jellyfin/emby and serve to anyone you like without all the networking or maintenance or really security issues.