@fh4pres  also, I heard/read about "Ethernet backhaul" but I have no idea if that even applies to my setup (maybe it does?)

@Tony thanks for the explanation!

Following up on this:

it sounds like I want to keep the Deco system in "AP mode" and set up "ethernet backhaul". This would provide maximum flexibility of moving the different Deco units around the house to different rooms with ethernet ports (which all go back to main 24-port switch in the data closet), as well as preserve the features provided by the Deco system (AV, parental controls, etc).

In case I decide to switch the Deco system to "Router mode", I would need to disable routing on the existing router (I think it is called "IP Passthrough"?). For that, I will contact my ISP tech support. Once that is done, would the wireless devices served by the new Deco system (tablets/laptops, phones, ring doorbell, etc) no longer be able to communicate with the ethernet-based devices (desktop PC, Synology NAS, smart TVs, etc) that are currently connected to existing router via the same main switch, since they would be on separate networks/subnets?

@fh4pres 

The drawback of AP mode in the Deco is you are pretty much left with the wireless coverage and mesh capabilities, meaning you can walk form one node to another and it will just switch over. The features like HomeCare would be disabled, here is a FAQ that talks about it: https://www.tp-link.com/us/support/faq/1842/

Now if you want the Deco to run in router mode, you will just need to place one of the Deco units directly to the router, and then place the switch behind it or have it so that the other two Deco units will not go to the AT&T, but to the Deco over the network. Using a Ethernet cable does give you the best possible performance, but if wiring will be too much of a pain you can just disconnect the Ethernet cable from the two slave Deco units and they will connect via Wi-Fi.

So maybe something like this:

                                                                                  <------ Switch <----- Deco

AT&T Residential Gateway <-----Deco <----- Switch <------ Switch <----- Deco

If you go with the router mode route I would leave the AT&T device as is, mainly to see if your network works well as is. If you notice any issues of services possibly being blocked or any issues arising then look at setting the AT&T as a bridge where it turns into a dumb device passing the internet to the Deco.

@Tony this worked perfectly!

Thanks again!