Itīs easy to fix, just put your ISP's IP (public or private depends of your ISP) , ISP Gateway AND Submask(255.255.255.0) into the Static IP configuration from your DIR-655 and just that, your NAT itīs open.
Before you do that, check if your ADSL or Modem has the option IP Passthrough enable with you routerīs name.
This is the config of my router DIR-655:
IP: 201.200.234.145 (default ADSL IP Public)
Gateway: 201.200.234.144
Submask: 255.255.255.0
DNS1: 192.168.1.1 (ADSL IP)
DNS2: 0.0.0.0
Regards!