Motorola surfboard SB6121

I would recommend that you do the following as a minimum:

1. Power-down the modem, AirPort base station, and computer(s).
2. Power-up the modem; wait at least 10-15 minutes to allow it adequate time to initialize.
3. Power-up the AirPort base station; wait at least 5-10 minutes. Note: The AirPort's status light may continue to flash amber after it has intialized. That is because, there may be some additional configuration items necessary, like setting up wireless security, before the overall setup is completed to get a green status.
4. Power-up your computer(s).

If the above steps do not solve the problem, start over with step 1 above, but then perform the next steps between steps 1 & 2. above.

1. Disconnect the AirPort base station from the Internet broadband modem.
2. While all of the devices are powered-down, perform a "factory default" reset on the base station. This will get it back to its "out-of-the-box" configuration and make setting it up much easier, especially if you use the "Assist me" process within the AirPort Utility. (ref: Resetting an AirPort Base Station or Time Capsule)
3. After the base station resets, go ahead and power it back down.
4. Reconnect the AirPort base station to the Internet broadband modem. For the Extreme and Time Capsule, be sure to connect the cable to the base station's WAN (circle-of-dots) port.
5. Continue with step 2 in the first set of steps.

In this basic configuration, the AirPort base station will broadcast an unsecured wireless network with a Network Name (SSID) of Apple Network NNNNNN. Network clients, connected to the base station either by wire or wireless, should now be able to access the Internet through the ISP's modem. Once Internet connectivity has been verified, you can use the AirPort Utility to configure the base station for wireless security and any other desired options. Please post back your results.

I have an SB6121 connected to my Extreme AC (also on Comcast) and it works flawlessly. I would check out the steps that Tesserax outlines. Since you said you plugged the cable modem directly into the iMac, make sure you keep the cable modem unplugged from power for a few minutes to give it a chance to reset and forget about the MAC address of the iMac, otherwise it may not connect properly and associate with the extreme when you plug into the extreme.

My airport extreme had worked perfectly with my old modem, but I could not get it to connect with the Surfboard SB6121. No one I called could help and I was frustrated to the max and just started messing with the router's settings. For me, the AE was set to "Off (Bridge Mode)" for it's Network - Router Mode. Once I switched that to DHCP and NAT the connection to the modem was made instantly and my internet began working (for half hour now...) flawlessly. Worth a try as it took 2 seconds.

Best of luck.

Just had the same issue and this fixed it! Great call on the "Off" mode and DCHP settings change!

This completely solved my problem with my Airport Express! I spent a week fiddling with the modem, calling my ISP, getting a service request. But changing the setting from Bridge Mode to DHCP and NAT did it in 2 seconds.