An external IP address identification service to learn the public-facing IP address of the server hosting the game. This is needed in order to generate the invitation links. If this service is unavailable Foundry would still work, but the invitation links would not be generated properly. Overall...
If you are trying to test if port-forwarding is working by connecting to the 'internet' invitation link from your hosting computer, it will commonly fail as most routers do not provide support for this type of connection. Try using a smartphone through cellular data, instead. An example of ...