Squad server status checker
Squad registers with Epic's online services rather than answering a query directly, so this check asks Epic about your server rather than asking your server.
Which ports a Squad server needs
That changes what a failure means. Port forwarding still matters for players connecting, but the result here depends on whether your server successfully registered with the backend — which needs valid credentials in its configuration, not an open port. Note also that this route does not return a player list, so an online server shows without individual players.
If it still shows offline
If the check fails, look at the server's licence and backend credentials before your router. If it succeeds but players cannot connect, the reverse applies: registration is fine and the game port is not open.