I got mine Thomson router working by port forwarding Gamespy and ARMA ports.

so go to "Game & Application Sharing" on your router and there should be already default Gamespy ports "Gamespy Arcade" and add it to your computer and then manually add ARMA 3 ports (2302 - 2309).

I tested my old Zyxel router and i connected to all servers right away. So for me it was the Thomson router that gave me the problem. But i'm sure they have to fix that problem. Because it cant "only" be the routers problem that it doesn't work, because if thousands of other games work fine with my thomson router but ARMA games don't, then Bohemia have to do something about....

I have also Thomson router and this is second game where i have had same problem, other game was Company of Heroes.