Having an issue with Double NAT?

So I have found out that my router and modem are in a Double NAT, with my modem giving router it's own private ip instead of letting it use my public ip, which brings up the issue of me not being able to port forward. So I tried to connect to my modem and see if I could configure it to DMZ all ports to my router so I could resolve this issue, but when I tried to load 192.168.250.1 (my modem's ip on my local network) nothing came up! Could it be possible that my modem doesn't have a configuration page? If so, how will I be able to fix this Double NAT problem? Thanks!

For the record, I have a D-Link 254 router and a modem from my WiMax service.