Hey, Ruby,
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1. You have to port forward Port 5400 TCP and UDP in your router`s settings for the PC that runs the server (your internal IP 192.168.1.21).
2. Your external IP adress (your "official" adress in the internet) changes daily (when you tested, it was 217.131.125.156).
3. You have to write this external IP in the client's config.json. For a game session, you can look it up in your router's settings, rewrite the .json and send the rewritten config.json to a friend to see if they can connect to your server.
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Hope I could help.