(Not really an answer but a few things you can try)
When accessing from outside your local network, do you put in your router public IP address? (And then assuming you set your router correctly, any request to the router public IP will be redirected to your NAS)
Also do check your firewall (on Router, and on NAS).
Router may have its own firewall, so maybe try turning that off, and also try setting DMZ to your NAS just to eliminate some blocking/firewall issue. DMZ is pretty much "any traffic I have no idea about, I will simply forward to this IP" so if your router is blocking something, DMZ will allow that bypass.
Also your NAS (Debian Squeeze) may have its own firewall and blocking request from your router / public IP as it is not an expected traffic. So you can try turning that off temporarily.
Once you can connect to the NAS box from outside your LAN, you can raise the bar slowly (firewall on router, and firewall on NAS) and see which one does the block. And once you found out which one, make a rule to allow that incoming traffic.
Hopefully it helps.