EAX12 doesn't see and won't connect to my 5ghz network

I have AT&T Fiber with a BGW320-500 router/gateway. It broadcasts one SSID for both networks allowing for the best connection for each specific device. I have a couple of dead (or very poor) wifi spots in the back of my upstairs, so I bought the EAX12 (AX1600) to try to get farther back there since the gateway is downstairs. While setting it up, it does not see a separate 5ghz network to use and 2.4ghz is connecting only. Although it does reach the dead zone pretty well when placed half way from them to the gateway, I am wondering why I cannot at least set up the unit and get it to use 5ghz since we are talking mesh compatibility. 

If I skip through, I get the 2.4ghz connection only on the status page. If I enter the 5ghz network name (same SSID as the 2.4) and password manually, it says it cannot connect to the 5ghz network. I assume this has something to do with the smart switching inside the BGW320 that won't allow one device two connections or some sort. 

If I deselect the 2.4 network and manually enter the 5ghz network, it doesn't connect to the router but still creates a network with the same SSID as my existing network as I can see with Wi-Fi analyzer.

Any tips or tricks to make this work, or maybe I have to split the two gateway networks into two bands to connect to each?