There's no reason either configuration wouldn't work.

I'm running all Deco's in AP/Ethernet mode back to a Netgear switch and it works brilliantly.

I don't know why it would be slow as it is, unless the ATT router doesn't support IEEE 1905.1 which is a requirement for Deco on Ethernet.

You can use any switch that supports that protocol - I think pretty much everything does except (cheap?) D-Link switches.

This is my current setup. On the first floor I get considerably slower connection compared to 2 floor. I think Deco 3 is using wifi backhaul instead of Ethernet. 

Have you tried swapping 2 and 3?

Or resetting 3 and setting it up again.

Yes, this system really needs an Ethernet backhaul indicator/test.

@121gtr Hi, have you maybe solved your issue? I will be trying to implement similar topology so I'm just curious. Regards.