Ah it’s not because you are private (mostly all of us are behind NAT so private, and that’s why the NAT Traversal library we coded) - it’s this:
Symmetric NAT with non-uniformly changing port mapping detected. No logic for Udp external address prediction for these circumstances!
That’s a very adamant router trying to defeat Holepunching and currently we don’t have a logic to beat that so IGD (if the test provided) would be the only option here.
It’s basically allocating random ports so that a prediction algorithm (like in the p2p crate) is defeated.