New Nighthawk X6S (R7900P) won't connect to internet

[Kaiger]

I just picked up the router yesterday at Costco and cannot get it to connect to my cable modem. I've got an Arris SB6121 modem and my ISP is Cox. The internet works fine with my old router as well as a laptop computer when I connected directly to the modem, but I cannot get the Nighthawk's "Internet" light to turn on for anything. The setup wizard says something about the ethernet cable is unplugged. I spent a couple of hours with Netgear tech support last night with no luck. Here's what I tried:

- Power resets (both modem and router about 20 times, letting modem fully boot before turning on router for most of them)

- Updated firmware to latest available (1.2.xxxx)

- Cloned MAC address from laptop

- Ethernet loopback test from LAN to WAN port. This turned on the Internet light amber for a second or two before it extinguished.

- Plugged internet cable into one of the LAN ports

- Factory reset on cable modem

- Cable modem reboot remotely via Cox website

- Different ethernet cable

After all this I decided it was a hardware issue, so I had my wife grab a second router. It shows the same behavior. I've googled everything I can think of and I haven't found any troubleshooting steps that I have not yet tried. I had no issues with the connection when I re-installed my old router. Any ideas?

[Kaiger · Accepted solution]

For anyone who finds this thread in the future replacing the cable modem did the trick. Had it working within minutes of getting the new modem provisioned.

[michaelkenward]

If the Arris SB6121 is a modem/router, you may have to work out how to set it to "modem only" mode.

If it is still trying to be a router, it will fight with the R7900P for control of the network.

The Arris manual says it has instructions on how to set it up for a router. Did you find that bit?

ARRIS Consumer Care - SB6121

[Kaiger]

I wasn't aware there was a combo unit that shared the model number, but my modem is a modem only--no router component at all. It's this one. The only thing I can even control on the configuration is to factory reset it. Also even though the modem manual claims it has instructions on how to set it up with a router it does not. It only has instructions for various operating systems. It is an older modem, so I could try a newer one.

Thanks for the reply.

Edit:

You've got me down the path of troubleshooting the modem now, which makes sense since I doubt I'd get two bad routers in a row. Another forum suggested that user disable IPv6. I'm not home to try it, but does that sound feasible? I purchased my modem in 2013, so I kind of doubt IPv6 was universally supported back then. 

[Kaiger · Accepted solution]

For anyone who finds this thread in the future replacing the cable modem did the trick. Had it working within minutes of getting the new modem provisioned.