Unfortunately, I won’t be of much help. I’m not familiar with WebRTC and may steer you in the wrong direction.
I believe that WebRTC signalling is based on websockets and you may be able to build your own server.
https://www.webrtc-experiment.com/docs/WebRTC-Signaling-Concepts.html
There’s been several other implementations using cloud-based servers. For example this one:
There was additional discussion here:
As far as I know, no one was able to implement an on-premise private signalling server. I’m not sure if it is difficult.