Quick answer: Zomboid timeouts are almost always UDP 16261/16262 or the server is still loading.
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Frequently asked questions (FAQ)
Timeout even though it runs?
UDP 16261+16262 in UFW and host firewall. Try direct IP, not only Steam list. Server often needs 1–2 minutes after start.
LAN works, internet does not?
Public IPv4, no NAT without forward. Panel shows the connect IP.