Valheim server status checker
Valheim listens on a small run of consecutive UDP ports, and the one that answers queries is not the one you type into the game.
Which ports a Valheim server needs
Players connect on the game port; the query answer comes from the port immediately above it. Forwarding a single port therefore gets you a joinable server that no checker or server list can see. Both need to be UDP.
If it still shows offline
Open the game port and the one above it, then retest. If the server is set to run without being publicly listed, it will not answer queries at all by design — that is a configuration choice rather than a fault, and a direct connection will still work for anyone who has the address.