I have a modem in front of my Asus RT-N66U.
It's on network 192.168.2.x, and sets the Asus (which is 192.168.2.2 on that connexion, but 192.168.1.1 to the rest of my home network) as its DMZ.
NATs not an issue - the Asus handles it all, the Billion modem just passes things back and forth.