Check if your Arma Reforger server is online
Unlike most games here, Reforger stays silent to this check until you have deliberately switched its query protocol on.
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The ports Arma Reforger uses
A2S querying is off by default; you enable it in the server config and give it a port. Until you do, nothing is there to reply, however your router is set up. Reforger's query also leaves out the player list, so an online result shows the server but not who is on it.
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Still offline? Try this
Turn on A2S in the server configuration, restart, and forward the port you gave it. If you would rather leave it off, the port tester can confirm the game port instead — that works either way.