@Turrash If the Deco receives 192.168.1.78 as WAN IP address (showed in the Deco app in "Internet Connection", I believe the ATT box does not behave as a bridge.

Could the ATT box run in "transparent" mode, assigning the public IP address to a unique MAC address, the MAC address of the old router?

Could you look into the list of clients of the ATT box, and look as well in the configuration of its bridge/transparent mode?

It is possible to assign a given MAC address to the WAN interface of the Deco, to let the Deco use the MAC address previously used by the old router: in Deco app, More / Advanced / MAC Clone / Use Custom MAC Address.