Help with xbox live and strict "NAT"?

Hey everyone I have a Thomson TG782T router and I have my xbox plugged into my wireless computer ( My router is upstairs and i'm downstairs) Also I bridge connections on my computer to connect. I have always had open NAT on my xbox but recently it went Strict out of no where and know I can't change it back. I have tried port forwarding done it all properly reset everything but nothing is working? Does anyone have any idea on how i could fix this because I really hate strict NAT! I heard you can use DMZ but i don't know if my router has that? Any help would be hugely appreciated!