Insurgency: Sandstorm server status checker
Insurgency: Sandstorm uses a query port that is separate from the game port and is easy to overlook, since the default is not adjacent to it.
Which ports a Insurgency: Sandstorm server needs
Both are UDP. The query port is what the server browser and this tool read. Sandstorm also uses an RCON port, which should not be exposed to the internet — this tool will refuse to test ports in the ranges typically used for administrative services.
If it still shows offline
Forward the game and query ports as UDP and retest. If the query answers but the server is missing from the browser, check that it is running with a valid game server login token, which listing requires separately from reachability.