Unturned server status checker
Unturned answers a Steam query on a port derived from its game port, which catches people who forward only the number they typed into the config.
Which ports a Unturned server needs
The server uses a small run of consecutive UDP ports starting at the configured game port, and the query is answered on one of the ports above it rather than on the base number. Forwarding a single port produces a server that is joinable directly and invisible everywhere else.
If it still shows offline
Forward the range rather than the one port. If the query still fails, check whether the server is set to LAN or internet mode — a server started in LAN mode will not answer external queries regardless of your router.