I have a BT Smarthub 2 and got a very similar message to yours when I first tried to set up ARC. For whatever reason the BT hub does not seem to do the autoconfig thing. So the next step is to manually set up Port Forwarding within the BT hub web access pages (should be on 192.168.1.254 via a browser). Once I did this it worked just fine.
So, have you already tried to do port forwarding in the hub?