Hope you are doing well !
Most likely, when changing the ATT Device to a google modem, being only a modem, it does not fulfill the same functions that the ATT router had.
Basically to make this network work you need one of the following setups :
Modem > Router > ICX-7150 C12P
Modem/router combo > ICX - 7150 C12P
Here is a short guide with most common Setups for Lennar homes users so you can review it :