Help Setting Up AT&T Gateway with Netgear Nightawk C7800

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hquan2001 wrote:

I used to have COX internet where the coax cable was used, now that I have upgraded to AT&T Fiber internet, I had to use this modem and ethernet combo to my Router.

This is classic "double NAT" territory.

Two routers on your network can cause headaches. For example, you can end up with local address problems. Among other things, the other router can misdirect addresses that the Netgear router usually handles, such as routerlogin.net or the usual IP address for a router, 192.168.1.1.

This explains some of the other drawbacks.

What is Double NAT? | Answer | NETGEAR Support

It may be possible to put the modem/router into modem only (bridge) mode and then to use the second device as the router. Sometimes it is easier to put the second router into AP mode. But that has its own drawbacks:

Disabled Features on the Router when set to AP Mode | Answer | NETGEAR Support

The bad news is that the C7800 is a cable modem/router. Unlike DSL modem/routers, these do not support AP mode. You may be stuffed.

While many questions about routers are generic and could be answered anywhere, some things need specialist knowledge.

You might get more help, and find earlier questions and answers specific to your device, in the appropriate section for your hardware. That's probably here:

Cable Modems & Routers

I will ask the Netgear moderator to move your message.

In the meantime you could visit the support pages:

Support | NETGEAR

Feed in your model number and check the documentation for your hardware. Look at the label on the device for the model number.

You may have done this already. I can't tell from your message.

I mention it because Netgear stopped supplying printed manuals and CD versions some years ago and people sometimes miss the downloads.