Port forwarding is something you would have to configure on your router to enable packets sent to your WAN IP (assigned by your ISP) to be routed/forwarded to your gameserver (setup in your LAN). Nothing to do with your config.cfg. You would have already done that for your game server...
Device Host: (note: Changes to the above 2 settings require Arma relaunch to be applied, as for server same for client machines;) Create a simple Notepad document called "CONFIG_server.cfg" and save it to the root folder of your Arma 3 install on the serverD:\Games\Arma3\A3Master ...