I absolutely understand what your issue is, and that is why I included the "double-NAT scenario" in my guide and have talked serveral people through the steps via my TeamSpeak server.
In my previous reply you are in the number 2 situation:
In practice: Modem (NAT protected) --->forwarded ports---> Netgear Router --->automatically allows all traffic---> PC (and other devices)
There is a fundamental difference between allowing traffic past NAT devices, and actually forwarding all traffic on certain ports to a specified endpoint (ie. your PC's IP)
The latter is what SINS requires. In your setup, the traffic is being forwarded
past one NAT-device, but then stopped at the second NAT-device (you're only using your second router as a switch, but from a networking perspective
it is still set up as a a "router"). Once you follow my instructions and also forward
the traffic from the Netgear on to it's final endpoint
(your PC) you'll have everything working nicely!
By the way, you can assign a static IP to your PC (as long as it's not within the range of IP's used by the Netgear's DHCP server) and this won't disrupt anything you've already got set up. That way the IP you're "forwarding" traffic to won't change on you one day...