Linking domain and remote access

[TheDreadPirate]

The new router or the NAS? If you have the option to run Jellyfin on a proper computer, do that over using your NAS to run it. Most NASes don't have the power transcode more than a few streams. And are particularly underpowered for HDR tone mapping.

There are also complications with the underlying NAS OS running older Linux kernels.

If you're talking about your router, maybe. Sounds like a pretty garbage router they provided you. If your router doesn't support IPv6, maybe it isn't properly port forwarding?