@UrbanB Hello, 

1.Please factory reset your powerline unit by tpPLC Utility one by one:

https://www.tp-link.com/support/faq/1612/  

2. Put 2 powerline unit in the wall sockets, in the same room, and pair them again :

Paring Note:  press Pair button only for 1 sec, Power Leds should be flashing on both powerline units in pairing process. ( if the Power Led does not blink, press it for 1 sec again)
When the Powerline Led or House Leds are on, the pairing process is done

3. Connect main powerline unit to Deco via Ethernet cable, and put powerline extender in another room 

4.  Download this tpPLC Utility and customize wireless settings for powerline units.

 https://www.tp-link.com/support/download/tl-wpa4220-kit/

Here is an FAQ to change a WIFI name and password for WiFi powerline unit as Deco's WiFi: 
http://www.tp-link.com/en/faq-1611.html