D-Link DIR-655 :: Change Its IP Towards Modem?

I've searched high and low across the wide web, but I have found no real answer to this problem;Recently, I bought the DIR-655 because the gods of thunder decided to euthanize my old WAP. The original plan was to mate this up with a Tilgin VOOD-modem (modem and router in one; a heap of s**** in my uneducated eyes) in order to make networking within my appartment somewhat easier.

Got it safely indoors, plugged it in as the manual told me, and got it going (my old WAP was also a D-LINK, so the interface was somewhat familiar). Checked cabled connection, worked fine, and I got the wireless-bit to function as well. But then I, or my Xbox 360 to be right on the money, encountered a little problem. As I understand from a little bit of research, I found out that this was a dreaded phenomenon known as

"Moderate NAT"

Before I got the old WAP indoors, I used the onboard router in the TILGIN to do the switching. I used to have this problem back then also, but solved it by opening some of the ports on the modem. Then it switched to "Open NAT", and worked like a charm. Now however, I have ran into a problem I can't dig myself out of. I have tried DMZ, opening numerous ports all the way to the full monty, but I can�t get that NAT to wander into the more acceptable range. And then it became clear that the internet on my laptop was somewhat amputated as well; simple youtube-videos became harder to load (sometimes it actually had to stop for extended streaming!), and this is not acceptable on a 20 mbt DSL-line!