DMZ keeps resetting ?

Hi,

I wanted to put my ps3 into a dmz so that I could get my nat type on mw3 to read open. After some effort I set my ps3 a static ip and assigned dmz to that ip. It worked and my nat was open.

However after turning on the PS3 the next day I found that my nat type read moderate again even though I never changed anything. Again I went ahead and managed to change it back.

The next day the same thing happened !

I never had this problem with my linksys router which clearly is a superior brand.

I have a n-300 adsl router.

Please help me...

Thanks