Check if your Valheim server is online
Valheim occupies a short block of consecutive UDP ports, and the number you enter in the game is not the one that responds to queries.
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The ports Valheim uses
Players join on the game port, but queries are answered one port higher. So forwarding a single port gives you a server friends can join and no checker or listing can find. Both ports must be UDP.
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Still offline? Try this
Forward the game port together with the next one up and test again. A server configured not to be publicly listed will not reply to queries at all — that is deliberate, not a fault, and anyone with the address can still connect directly.