Arma Reforger server status checker
Reforger will not answer this check at all unless you have explicitly turned the query protocol on, which is the opposite of most games here.
Which ports a Arma Reforger server needs
The A2S query service is disabled by default and has to be enabled in the server config, with its own port specified. Until that is done there is nothing listening to answer, no matter how your router is configured. Note also that Reforger's query does not report a player list, so an online result here will show the server without individual players.
If it still shows offline
Enable A2S in the server configuration, restart, then forward the port you assigned it. If you would rather not enable it, use the port tester to confirm the game port instead — that works regardless.