Check if your Project Zomboid server is online
Zomboid runs on UDP and keeps the port Steam talks to apart from the one direct connections use, so the result you get depends on which of them you test.
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The ports Project Zomboid uses
Out of the box there is a direct-connect port and a Steam query port, with different numbers. If just one is forwarded, the server may work through the in-game direct-connect option and still be missing from the public list. All of it is UDP.
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Check that both are forwarded as UDP and give the server time to finish loading — Zomboid rebuilds a lot of world state at boot and ignores queries until it has. With mods, a version mismatch that stops the server from starting looks exactly like a closed port from outside, so read the server console before you blame the network.