Using ATT BGW210 and Netgear RAX20 Simultaneously

While you will not get features such as 802.11v and r  with such a setup, if you put the Netgear router in AP mode, and give it the same SSID and password, then you will have an overall single SSiD andd roaming will be handled entirely based on the client devices roaming decisions. Most clients will roam based on RSSI values set by the WiFi drivers, e.g., some may decide to roam at -70dB while others may look to roam sooner.

From a networking standpoint, a having multiple different WiFi routers just set up as APs, with wired connections back to the main router, will function fimilarly to modern mesh systems, though the APs will not assist in the roaming process, such as directing a client to roam to a specific AP.

With a wired backhaul, performance will still be very good for each AP.

The only time you will run into some annoyances is if you are using a very old WiFI device which tend to be sticky clients that won't roam unless the client device has too many corrupt frames as the lowest MCS index, for newer ones, they tend to be more well behaved in not letting things get that bad. Forthermore some newer devices using low end and obscure WiFi chipsets, also tend to be bad with client directed roaming.

If your devices are fairly modern with decent quality WiFi chipsets, then setting the RAX20 in AP mode, will offer you a mesh-like experience for those devices (as long as you give both wireless routers the same SSID and password) The response times between changing locations and a roaming event will just be slower than on a true mesh system when various roaming assist features to direct and speed the handoff.