Works on same network but not on diffferent networks

Hi there

When spinning up webrtc-cli on one machine and your HTML sample in a browser on another machine on the same network, audio flows fine.

When one machine is moved to a different network, the audio never connects.

Both networks are behind NAT, one is my home internet connection, the other is my laptop tethered to my phone using its 4G.

I notice you can specify a STUN server in webrtc-cli, but [the internet suggests](https://stackoverflow.com/a/47910051/17971) this may not be enough, and TURN might be needed.

Any ideas? I can grab wireshark traces if helpful.

Cheers.