Hey! I happened to stumble upon your game after finding some examples of multiplayer games using godot, particularly with WebRTC. Coincidentally, I also participated in this jam though! Shame I didn't find your game earlier... Anyway, I've been trying to learn about networking in godot for a few weeks now.
But I wanted to say, I think I know why some people people are struggling to connect to other people's servers.
My first thought was that maybe it was because of people's particular network setup, but it seems like you did all the right things on the client-server side to get around that. Unless people are on some business network or something, there really shouldn't be any issues here, I would hope.
So, what I actually think is going on is the fact that your using Heroku to manage your server. The problem there is that Heroku is doesn't run your server code on a single machine. It could be several. But your client-server interaction assumes that the server code is on a single machine, since you're storing all the information about peer connections locally on the server.
I imagine that the way this is normally done is to have a single dedicated server that everyone has to interact with in order to connect to other peers. But I don't really know if that's possible on Heroku.