First, thank you in advance.

I'm having trouble with exposing my server, I think what I need is a better understanding, as opposed to technical help (though that would be appreciated)

At the moment I'm using the linuxserver.io suite of applications. I've got SWAG set up with DuckDNS, and I'm trying to set up Jellyfin and other applications. (they're all in the same compose.yaml).

I can access my applications on an external network via <user>.duckdns.org:<port> and it works fine (but no https).

Within my home network I can access jellyfin.<user>.duckdns.org - the https is valid and everything is working fine.

I suspect this means my router is not set up correctly? I'm using OpenWRT. What am I doing wrong?