So I have lived in South Korea for 6 years now. The fact that this fire has had such a major impact is quite typical of Korean bureaucracy and tech administration. Very few backups, infrastructure held together with scotch tape and bubblegum, overworked devs and maintainers. It's a bit sad, especially for a country that exports so many tech products.
Tailscale creates a new virtual network, and anything you want to connect to the Jellyfin server needs to be in that Tailscale network. You need to hook up the Roku device to Tailscale. I have never owned one, so I can't say anything about that. But maybe that's what you should figure out next.
I updated my Dietpi setup today, because a new version was available. It went very well, and everything works perfectly after a reboot.