NAT type switching on me?

Okay, so I have a 360, obviously, and my NAT type has been switching on me. My friend showed me what to do to change it from Closed to Open. I did that and it worked. And when my brother came home from college with his new Xbox and plugged it in wirelessly and started to play COD Black Ops, my NAT type has changed. But when he was offline, on the xbox, it went back to Open, from like Moderate. I reset all of the stuff that my one friend told me to do, and redone it. It still won't go back to Open. But I have a Verizon DSL Model 7500, and what I done in the Router settings was went to the, "My network" tab and clicked on my Xbox, and clicked Enable Application next to it. And added the service, "Xbox Live" as a Host. And when I put down for what LAN device it uses or whatever, I used the computer that I am on and the only computer that I have on this LAN. And then restarted the Modem and Xbox and it still wouldn't work. Is there something that I did that makes it change on me when other people, like the Xbox users, connect to the network? I would be pleased for anyone to help me.

Oh, and I forgot to add this, I also went to the Advanced tab and clicked on Universal Plug and Play and enabledUPNP. And I disabled it as well when I added the Xbox live service. Like I added the service with UPNP off and on. Both. And it still didn't work. But yeah. I would like help... Or any suggestions..

This used to happen to me as well. You might want turn off your modem and the turn it back on to see if it gets fixed. If you think you can fix it your self without a problem here'sa guide that helped me make my NAT Open.

My router also switches from open to moderate every few days. I just unplug the power wire on the back wait 10 seconds and plug it back in. After doing that the status always goes back to open...at least for 3 days :P BTW this tip was suggested to me by my xbox after running a network test. It simply told me to try resetting my router. If I was more comfortable trying to fix it for good I would, but my network works great so I don't wanna mess it up :P