@tinosa606,
There are a few threads on the forum with people detailing their setups with that modem, however there is not much to do.
The most important thing is in regards to your ATT BGW210 - This is a Gateway Device, which combines a router and a modem into one unit. To use this with your Deco system in Router Mode(All Features) you must place the modem into bridge or IP passthrough mode, which will remove the wireless functionality from the device.
If you do not enable this, you must setup your Decos in AP Mode(Limited Features)
If you are making the transition to a new provider, I cannot recommend enough to get a modem that has no routing capabilities. This way, the device will be entirely devoted to the connection rather than having half of the functionality disabled. Your ISP should have a list of compatible modems and may even provide them in exchange for your BGW210