I read in some online articles that I will need to change some settings on the BWG gateway.  Do you know if that is true?  

@JHouse 

The ARRIS BWG210-700 does have two methods of configuring a type of Bridge Mode.
First is "Default Server Mode", the preferred method is "IP passthrough mode"
You can just set it in the Arris Management page:
1.) "Home Network" Tab, "Wi-Fi" Sub Tab, Click on the "Advanced Options" link.
Log in with the Device Access Code as printed on the Arris Modem Label.
2.) Turn off both WiFi Bands by turning OFF the SSIDs.
3.) Go to the "Subnets & DHCP" Tab - configure the Assis Private LAN Subnet
to something far away from the Subnet that your Deco is using. Click SAVE when finished.
Log in to the new modem IP address with Device Access
4.) Go to the "Firewall" Tab, "IP Passthrough" Sub Tab
- Change Allocation mode to "Passthrough"
- Passthrough Mode should stay "DHCPS-fixed"
- Choose the MAC address tor your Primary Deco Node.
- Set the Passthrough DHCP Lease life to 99 days
SAVE and Reboot all TP-Link Deco Nodes currently connected.
5.) The Home Network Tab - Status Tab should now list "IP Passthrough Status"
On (public IP address) and the External IP ISP address should be listed as "IP Passthrough Address"
6.) Go to Deco App and look at the Primary Node Internet Status:
The Internet IP address should now be the same as the "IP Passthrough Address" on the Arris.

Thanks so much for your detailed response!

@JHouse 

If you want the Deco to run as an access point where it is not a router, then leave the Motorola as is, and connect one Deco to it, set it up, and switch to AP Mode: https://www.tp-link.com/us/support/faq/1842/

If you want the Deco to be the router and the Motorola to do nothing other than pass the internet to it then you would set the Motorola to bridge mode like @DAServices mentioned.