I recently switched to Time Warner's Business Class Internet Service in my home. Previously I had Consumer grade RoadRunner service also from TWC. In my original configuration I had the TWC Cable Modem to which I connected a Wireless Router from D-Link (DIR-600). I connected the Ethernet cable from the cable modem to the WAN port on the D-Link router. All was well, I could connect wireless devices to the router, and wired devices as well via the 4 port switch built into the D-Link. 
Now along comes business class service, which included a new Cable Modem/Router combo, which was an SMC8014-TWC device. I was not home when the installation took place, and when I got home the D-Link was connected to the new SMC router via an Ethernet cable from a switched port on the back of the SMC to the WAN port on the D-Link. Chaos ensued as neither of my children could connect to the internet and therefor were in imminent danger of having their social lives implode!
The D-Link could no longer reach the internet, and therefor neither could anything else connected to it. My solution was as follows and I hope it is helpful to someone out there.
The SMC device had an internal IP address of 192.168.0.1 with a subnet mask of 255.255.255.0 the D-Link, initially had an internal IP address of 192.168.1.1 with a subnet mask of 255.255.255.0
So first things first, I disconnected the WAN port on the D-Link, and connected my laptop to it via wired cable directly into one of the 4 switched 
ports. I then changed the Routers internal IP address to 192.168.0.2 (I also changed the DHCP address range it was giving out to be 192.168.0.100 to 192.168.0.120).
I reconnected the D-Link to the SMC by the Ethernet cable but this time not via the WAN port, instead via one of the switched ports. Then I went to all of the computers attached to the D-Link and made sure they were using 192.168.0.1 as their gateway address and 192.168.0.1 as their DNS address and everything came back up. I accomplished that by disabling and then re-enabling the local area connection on each which caused them to renew their IP configuration from the DHCP server.
Cheers,
Bobster