How to change NAT Type 3 to NAT Type 2 on PS3?

Hi, i've been trying to change the NAT Type on my PS3 from NAT Type 3 to NAT Type 2. I have assigned a static IP to the PS3, and opened the ports TCP:80, 443, 5223 and UDP:5223, 3478, 3479, 3658, however, i'm still getting NAT Type 3 and i can't seem to change it. My router is D-LINK DIR-655 if it matters. Will appreciate any help on the matter. Thank you.