When adding servers on DSub, it has the option to set a local network ssid and address for each media provider. So it can use the local network to access the server when you are at home, while still having access via a reverse proxy when outside.
Would it be possible to add something similar to Symfonium?
Sorry there’s no plans for that, Google require fine location permission to access SSID I refuse to add that.
Most router support reentrant NAT, and for the other cases DNS views / host masquing solve the issue. Everyone should have a pi–hole or equivalent for privacy at home 
And the last ressort is to add 2 media providers in Symfonium and enable the source filter only online provider that will ensure the switch between them. Symfonium cache is smart and image / offline file cache will be share between them.
There should be two IP addresses (like DNS) while setting up online media providers. That way if one address (local) is unaccessible, the app would try to access the second address