@YankoM,

With both running as routers, you would need to have two DHCP servers running, creating a Dual NAT - this would also not allow devices connecting to either Wi-Fi to see each other and would likely cause stability problems for devices across your entire network.

What you are looking for, is to run the Decos in AP Mode, which will remove the Routing functionality from the Decos, allowing your modem/router complete control over these functions such as addressing. Since many of the features of Deco, rely on being able to address the devices, you should read through this FAQ on the differences between router and AP mode. Many of the missing features would then need to be configured through the interface that AT&T provides for the gateway.

In AP mode you will still be able to configure the wireless settings to use a different SSID from the ATT Gateway, however keep in mind that having a mesh and non-mesh network may interfere with the seamless transitioning expected with mesh networks.

To setup the Decos in AP Mode see this FAQ: How to set up Decos in AP mode