I believe I'm portforwarding and everything just right, but I still get strict NAT.
This is the situation:
Tried plugging the Xbox directly and port-forwarding on the router, worked like a charm.
For practical use though, I need to plug the xbox with ethernet cable via PC to a wireless router. This is where the problem occurs.
I set static IP for the xbox, bridged the wireless connection on my PC (avoiding the double NAT problem, right?) , turned on ICS, and configured the router to port-forward to the static xbox IP. Also enabled UPnP. No firewall-settings on router up to cause interruption. Why oh why do I still get strict NAT? I do get internet connection.
Randomly, I also did manage to (briefly) get open NAT by portforwarding to the PC IP only, not the xbox (huh?), but then I was still not able to joining friends' parties etc.
For troubleshooting: I'm on win7, the router is a speedtouch ST780.