My experience setting up a TP-Link mesh network on BGW320-505 using Deco x55 devices
Hopefully this will help others. Feel free to edit. I am sure I left something out. :)
Download the TP app to a mobile device like a mobile phone.
1) With the router still connected, change the setting to Passthrough.
2) Turn off the 2.4 and 5 antennas.
3) Turn off ATT router's firewall.
4) Disconnect computer from router and connect the Deco X55 device.
5) Wait for the device light to turn to a flashing/pulsing blue color.
6) Follow setup instructions for your Deco device.
IMPORTANT: I wasted a couple of hours trying to set up the TP network to have the same SSID as the ATT router. MUCH easier to use a different SSID and change it back when set up is complete. And that way you do not have to change your devices' settings.