[anacrolix]

There's a less commonly known feature of NAT (maybe called cone NAT or something), where the remote endpoints are not checked for incoming packets and connections. You communicate with trackers and DHT and as usual your outbound packets are mapped to some port. But everyone use sees your public IP and port and talk about your client using that pair. They then also communicate with you over UDP to your public port. As long as you have regular outbound traffic running through that mapping, NAT will keep the hole alive. I think with testing I found at least half of people on NAT had this. It's less common on mobile and fibre.