Use the Multiplayer - Add ICE server action.
Add ICE server
Add a custom Interactive Connectivity Establishment (ICE) server used by WebRTC to establish connections between peers. There are a couple of built-in public STUN servers used, but you can also provide your own TURN servers to enable connectivity through certain kinds of NAT. A username and credential can also be optionally provided if the server requires them.
An ICE server is a blanket term for both STUN and TURN servers.
You want to put this action before connecting to signaling server.