You have 2x approaches...to flip your DLink router into basically a wireless access point.
Or...the approach I use for my clients offices setup on Comcast business...I use my clients own router as the only router, and set the SMC gateway to pass the public IP to the WAN interface of my clients router.
Since you have an SMC Gateway from Comcast..you probably have a static IP from them. They should have written down that info when they set you up...you'll have the static IP range, subnet mask, gateway, and DNS. If you don't have that..call them..they'll give it to you. Ask them what your first usable IP is (or just post here asking..but I'm not available again until Monday).
Now...plug a computer into your DLink...log into its web admin, setup your WAN interface to static IP..and enter that info. Now..uplink the DLinks WAN interface to one of the LAN ports of the SMC. Next...plug a computer into one of the other LAN ports of the SMC..and log into its web admin at 10.1.10.1 username cusadmin password highspeed or visa versa.
Go to the firewall tab...look around, there's a checkbox for "Disable fireawll for true static IP subnet" ....put a check in that...apply/save....and reboot the SMC. After a minute goes by...reboot your DLink. Now...plug computers into the back of your DLink....surf the net. Go to whatismyip.com and check the IP..should be the IP that you typed into the WAN port of the DLink. So now the SMC is basically routing (passing) that public IP you entered to your own router without any firewall/NAT features.