The default multiplayer plugin already includes some turn servers. Stun is what you're looking for
Actually it's the other way round: there are some built-in STUN servers, which can broker connections, but not link up devices with restrictive NAT. You need to provide your own TURN server, which acts as a relay and so has to retransmit the entire bandwidth used by the gameplay, to ensure devices with restrictive NAT can still communicate.