Project Zomboid server status checker
Zomboid uses UDP and keeps its Steam-facing port separate from the one used for direct connections, so which port you test determines what answer you get.
Which ports a Project Zomboid server needs
The default configuration exposes a direct-connect port and a Steam query port, and they are different numbers. A server reachable through the in-game direct-connect option can still be invisible to the public list if only one of them is forwarded. Everything here is UDP.
If it still shows offline
Confirm both ports are forwarded as UDP and that the server has finished loading — Zomboid rebuilds a large amount of world state on startup and does not answer until it is done. If you run mods, a mismatch that stops the server booting will look identical to a closed port from out here, so check the server console before blaming the network.