Technical bits: NAT/PAT firewalls read and track five items: source IP, destination IP, source port, destination port, and protocol. When one's app transmits a UDP packet toward the relay, the NAT/PAT firewall will track these five items and then performs PAT, which changes the source port just as it exits the WAN interface. The app never knows this, therefore the relay will track this IP:PORT combo so that it can return traffic back towards the user.