Alas, this is a general issue with WebRTC: Most people are behind NATs[1], and not even STUN[2] can save them. In order for this to support internetwork (heh) transport, you'd need a TURN[3] server that could relay the traffic between the participants. Of course, at that point you're not really Peer2Peer anymore: You have a server between you and that server is going to cost actual money to run.
[1] https://en.wikipedia.org/wiki/Network_address_translation [2] https://en.wikipedia.org/wiki/STUN [3] https://en.wikipedia.org/wiki/Traversal_Using_Relays_around_...