Atlas' dedicated server uses at least three ports, and three per shard plus a lobby server port if you run more than one grid square. You'll have a game port, a query port (which I suspect will be what Steam will use), an RCON port for administration, and a redis port for the lobby server if you have more than only one grid square.
Port Required for steam Stats - LAN Server
in Dedicated Server and Developer Tools Discussion
Posted
Whats the lobby port?