Connecting an ORBI RBRE960 Router to AT&T Fiber with ATT Gateway BGW210-700

You have a number of options which depend on how you intend to use the Orbi system.

The most basic is to follow the installation guide:

https://www.downloads.netgear.com/files/GDC/RBRE960/RBRE960_QSG_EN.pdf 

Load the Orbi 'app' on a smartphone or tablet.  Open the app and to connect the Yellow Internet port on the router (RBRE960 on the product label) to the AT&T BGW210.

Enabling Armor & Parental Controls is entirely optional.  (I have never done so after noticing "free trial" which implies that these are paid subscriptions.) For myself, I would rather use the Orbi web interface from a computer connected by cable to the Orbi router.  (provides access to options that the Orbi 'app' does not.)  See page 15 of the User Manual:

https://www.downloads.netgear.com/files/GDC/RBKE963/RBRE960_RBSE960_UM_EN.pdf 

Most customers would choose to disable the WiFi on the AT&T BGW so that there are not two WiFi systems competing with each other.

This basic installation comes with one potential issue: The "Double NAT" created by having two routers in series.  (Check some internet descriptions for information.)  There are specific applications that are affected by a Double NAT, such as using OpenVPN to reach your LAN from the internet, allowing internet access to servers on the LAN, and certain forms of internet gaming.  If these applications are not needed, then you are "good to go."

As you may have noticed, butting the BGW210 in Bridge or Passthrough mode is a complicated issue.  Internet search will turn up dozens of articles on the issue.  If you really need to do this, my guess is someone will offer suggestions.