Xbox One in DMZ but has strict Nat

[#1]

I have an issue where my Xbox one has strict Nat even though I have put it in the dmz. The only way to get an open nat is to reset everything, but it changes back to strict after a day or two.
Router is a Thomson 585v7 - can anyone advise?
Thanks 

[#2]

after you have reset router do you put console back in dmz? sounds like it could be getting different ip.check on router you have ticked always use same ip.

[#3]

Thanks, will try that