It's long but I wanted to try to provide all the info out so hopefully it covers all bases for advice. Thank you in advance! 

Before moving I had the older AT&T mesh devices (AirTies 4970) that I bought and they worked well on wired backhaul but when we moved AT&T gave us the newer modem/router (BGW320-500) and it's not compatible with those old AirTies so I bought the Decos because the new AT&T mesh devices are billed monthly with no option to buy outright. I don't want to have to pay monthly for a good connection. I set up the TP-Link Deco W6000, x2 units, a couple weeks ago. Everything was going great until the last couple days. At first I was getting improved speeds on Deco (500-600 Mbps down and 400 Mbps up on WiFi on my phone) vs AT&T BGW320-500. (400 Mbps up and down). But now from same spot next to either Decos on the same device (I checked with other devices too) I'm barely getting 100 Mbps down and 60 Mbps at any given time. Attaching photos for reference, note the screenshot of new device notifications of the Decos picking each other up after reconnecting.

My ISP speed is 1000Mbps up and down through Fiber and BGW320-500. It's pretty solid but wasn't able to reach all corners of our 2900 SqFt, single store ranch style home from the data closet. Any insight would be appreciated as I've tried the support steps and I'm a little disappointed the speed and connection has worsened after switch to these Decos versus the one modem/router our ISP provided. We have the 

The Decos are both setup wired backhaul so I believe the connection should be good and consistent. Main Deco is wired to the BGW320-500 into the 5G port, and nothing else is wired to the BGW320-500. The Main Decos second port is wired to the switch ( Netgear GS116E v2 spec for 1000Mbps ). From the switch I have the Second Deco wired and the other ethernet connections for the house.

Wife and I both work from home so we immediately notice the disconnections. It happens randomly throughout the day. When I look at the Decos, once the LED was red on the Second Deco but most often solid green and Main has always been solid green when I check. I also check the AT&T app and the BGW320-500. and that's also all good confirming it has internet connection through it's own app Speedtest. I check the switch and that's on with lights blinking with traffic. 

Questions;

• Are there any settings I need to be looking at on the Deco app or my BGW320-500.? I've changed the DNS on Decos but I'm not sure if I need to do the MAC cloning. Fast Roaming and Beamforming are on. DHCP allocation is on Auto on Deco and my modem. IP4 is on and IP6 is off. 
• Deco is setup as Wireless Router by default, should I consider setting up as access point? Does setting up as AP mean I can also use my modem/router's wifi with the Decos thereby give me three points of connection? 
• I can't really tell on the Main Deco, because the app is a little lacking, but I've notice a couple of time that the second Deco sometimes switches to WiFi vs Ethernet which I find odd. Is there anyway to disable the Deco from connecting through WiFi since it's hardwired?
• Is there a way to set the Main Deco to transmit only 5Ghz? I saw in the app an option but it gets applied to both Decos, I want to only set the Main to 5Ghz to try to minimize devices connecting to the farther Second Deco.
• Anything else you can recommend for me to try/check?  
• Should I consider one of the newer Decos/mesh devices? I know there are newer, better models but based on our ISP speed and our use I thought the W6000 would be a good budget point that could handle the use but now I'm wondering if I made the wrong decision.  ​​​​​