Sons of the Forest server status checker
Sons of the Forest uses Steam networking, so the ports involved are the Steam ones rather than a single obvious game port.
Which ports a Sons of the Forest server needs
The server needs its game port plus a Steam query port, both UDP, and they are in different ranges. The query port is what this tool reads. Dedicated server documentation lists both; forwarding only the first is the common error.
If it still shows offline
If both are open and the query fails, check that the server is running in dedicated mode with a valid save selected. A server that fails to load its save can stay running as a process without ever becoming queryable.