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Thats usually because you may have to manually set the ports in your router ..
In the launchers settings > MP Settings.. if the port mapping is set to manual, which is sets to that if it can not use/detect your routers UPNP (Universal Plug and Play) where it would set it for you if it did ..
Theres a small guide https://www.reddit.com/r/projectceleste/wiki/how-to-play-coop-pvp-aoeo-celeste-fan-project which may help, if you can not do the port forwarding in your router or cannot access it, you may have to use Hamachi instead, but also your friends will have to use it too ..
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