As mentioned, you cant manually set up your listening ports - this is handled internally by ICE, which needs to connect to a STUN server to get (at a minimum) your public IP address.

edit: cant reply to your comment fulafel, but on that project you posted: https://github.com/cjb/serverless-webrtc/blob/master/serverl... Also, note that even if this wasnt defined, some browsers contain defaults. You 100% need STUN, but you can handle the signalling manually - as I stated.

edit2: cant reply to your comment ibc, but I was explaining the bare minimum 'serverless' webrtc case still required a STUN server. I appreciate that mediasoup SFU uses ice-lite instead.