@Helldiver_M @Dubious_Fart @leraje actually the port forwarding thing is about accepting inbound connections. Without port forwarding, NAT routers (including VPNs) randomly allocate ports for outbound connections but still won't accept inbound connections on those same ports.

There's a trick where you discover the randomly allocated port numbers and then both connect to each other at the same time so both routers think it's outbound. It works unreliably and BitTorrent doesn't use it.