Orbi AC3000 want to connect to ATT uverse gateway (have a Arris NVG510 modem & Netgear R6250 router

[mcgk]

I want to connect a new Orbi AC3000 to my "network".  ATT Uverse as my internet provider.  I have a NVG510 Modem which has a CAT5 cable in one of the ethernet ports connected to the Internet port of my Netgear R6250 router. This was set up originally by ATT. This router has both 2.4 and 5GHz.  I am doing it to improve the wifi signal in our house. THe Uverse cable comes into our house upstairs on the northside of our house and we want to be able to stream on our TV downstairs in the great room on the southside of the house.  I need step by step directions. In doing a lot of reading on the web it appears no one has all of the steps listed in one spot.  I read about having to disable the 2.4 and 5 wireless networks on the current router. The network status of the ATT connection it shows "Interface Wifi status is disabled.  There is mention of changing the Orbi to AP mode.  I am in over my head and would appreciate any help.  Do I just unplug the R6250 cable and  plug in the Orbi router ethernet cables?  

[FURRYe38]

Modem Combo Units:
Couple of options,
1. Configure the modem for transparent bridge mode. Then use the Orbi router in router mode. You'll need to contact the ISP for help and information in regards to the modem being bridged correctly.
2. If you can't bridge the modem, disable ALL wifi radios on the modem, configure the modems DMZ for the IP address the Orbi router gets from the modem. Then you can use the Orbi router in Router mode.
3. Or disable all wifi radios on the modem and connect the Orbi router to the modem, configure AP mode on the Orbi router. https://kb.netgear.com/31218/How-do-I-configure-my-Orbi-router-to-act-as-an-access-point and https://www.youtube.com/watch?v=H7LOcJ8GdDo&app=desktop