Mesh Router with ATT modem/router

The 5268AC is a gateway (router/modem combo). So if you are going to use another router with it, then you should place the gateway in pass through mode. Which basically disables the router portion's WiFi and allows you to use the Orbi (Netgear's Mesh WiFi) as your router and WiFi. Otherwise, you can have WiFi issues. You can still use the ethernet ports on the 5268 for wired connections.

You can restrict access to the Netgear for certain devices.

Thanks for the "help" @OttoPylot, except....

The 5268AC uses DMZ Plus, not IP Passthrough. 

Enabling DMZ Plus (or IP Passthrough on the Arris gateways) does not disable Wifi on the AT&T gateway. You have to do that manually, by going into the gateway's UI and turning off the wifi radios for each band.

@ThoDmas  You didn't mention if you set up the Netgear using the DMZ Plus feature on the Pace gateway.  If you haven't, here's a link to how to do that (scroll to the bottom and look for the 2wire/Pace section)... https://www.att.com/support/smallbusiness/article/smb-internet/KM1188700

It's usually advised to disable the wifi radios on the AT&T gateways when you use a good third party router.  The thinking is that the third party router will do a much better job.  The state the Pace's wifi radios shouldn't have any effect on the mesh system's ability to stay connected, though, since your mesh system is hard wired to the Pace.

There's really no advantage to forcing your "roommate" to use the AT&T or disadvantage to them using the Netgear that I can think of.

If you have things like a Flex server, or are worried that he might be able to hack devices on the Wireless LAN, then I can see the arrangement of treating the 5268's WLAN as a "guest network."   It does mean another device broadcasting and receiving signals in what could be a crowded Wi-Fi situation.  If your Netgear has a Guest network feature, I would suggest using that instead and turning off the Wi-Fi on the Gateway.

In either case, I agree that:

1) You should turn on DMZplus on the 5268

2) Turning on the DMZplus will not implicitly turn off the Wi-Fi on the 5268

3) If you don't need it, then turning off the Wi-Fi on the 5268 would be recommended.  If you're going to leave it on, you should consider locking the channels on both the 5268 and your Netgear to something different (On the 2.4 GHz only use 1, 6, and 11, never 2-5 or 7-10; on 5 GHz, spread them far apart).

@tonydi thanks for the slap on the wrist. I'll try to be more careful next time with generic replies.

We have an AT&T BGW210 modem and a Nighthawk Mesh  WiFi 6 Router system connected and have placed the gateway in the pass through mode.  We are only able to get internet using the password to our AT&T network, although we see the Nighthawk as a choice of networks and can sign in but we can not pull up a website...I expect that is because it is in pass through mode.  How do we REALLY know that the Nighthawk is giving us strong WiFi signal.  I can be in the same room as one of the mesh 'boxes' and neither my iPhone or iPad show a strong wifi signal so I am skeptical.  If I am sitting in the same room as the AT&T modem and the 'home' nighthawk box I have a strong wifi signal.  Can you offer an opinion?

.I expect that is because it is in pass through mode

I expect that it is because either (a) it is not fully operating in IP Passthrough mode, (b) the Netgear is using the same subnet as the BGW210, and/or (c) the equipment has old IP leases that need to expire and be renewed on the Nighthawk.

To check (a), look at the WAN IP for the Netgear and make sure it's a public address, and not a private one like 192.168.1.xxx.  If it is private, unplug the Netgear for 5 minutes and plug it back in and see if that fixes it.

I will also add the general disclaimer that if you have never done a Factory Reset on the Gateway before setting up IP Passthrough, you might want to do that now, and then configure it.