I provided some solutions in the other thread pertaining to this same setup, so that may be a preferred solution, particularly if you prefer your own router to the one provided by the cable company.
But if you want to take yet another approach and just use the D-Link as a WAP, it's pretty simple. Just disable its DHCP server, give it an IP address in the same scope as the modem+router (of course, an IP address not in use elsewhere, statically or handed out by DHCP), and patch them LAN to LAN.