I'll explain this the best I can, if you're hosting something from a computer on your network and you connect to the world using the network IP (192.168.x.x) the pager will report that as your external IP and will attempt to connect people to that IP if they attempt to join you through the pager. Logging out and back into the world using the actual external IP fixes the issue.
What I'm guessing is the pager doesn't realize it needs to report the external IP because it assumes the person is connecting to a LAN game and the internal IP is all that it needs to handle.
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