Space Engineers server status checker
Space Engineers uses a Steam-based query on its own port, separate from the one players connect through.
Which ports a Space Engineers server needs
Both are UDP. The query port is the one this tool and the server browser use, and it is a different number from the game port by default. Forwarding one and not the other is the usual reason a running server is invisible.
If it still shows offline
If both ports are open and the query still fails, the server may still be loading. Space Engineers rebuilds the physics state of every grid in the save on startup, so a world with a lot of large ships can take a long time before it answers.