The NAT traversal logic here is really basic and won't hold up well in practice.

A good chunk of NAT devices will pick external port based on [src ip/port, dst ip/port] combo, not just [src ip/port], so "WAN IP/port" you get from STUN will get you nothing useful. Not by itself.

STUNs should be used for discovering the pattern in NAT port overloading logic and then using it to predict which port your peer will use towards you if you were to try and connect now.

That is, you need to know the overloading pattern and then also time stuff correctly.

For that reason you will need a rendezvous server and it's also the best to let the server drive the whole process (as opposed to what STUN-based setups do, which is to let clients do it).

PS. In my past life I made a P2P VPN called Hamachi, which used all this stuff very extensively.