How to install what configuation to use Model # RBR20 on base

[Snowtoyfj40]

I have an arris cable modem, I want to put the Orbi after that in wifi only mode and have it pass throught to a firewall wich is a router and wifi  with another router (netgear) to run my VOIP system. 

My question is what is the best way to put the orbi first in line after modem so it wont mess up other system wich is working great. I have read DMZ and change to a diffent IP adress . What would be the best configuation on th Orbi. Thanks

[FURRYe38]

What FW is currently loaded?
What is the Mfr and model# of the ISP modem the NG router is connected too? Built in router here? 

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.

If the modem doesn't have a built in router, then you'll need to connect the Orbi in AP mode to a router thats behind the cable modem.