Hello @Paulor81 

Please refer guide here to add extra powerline unit into existing powerline network by pair button:
https://www.tp-link.com/support/faq/1081/

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.

Or refer to this instruction to pair the powerline units/ add an extra powerline unit by Powerline Key: 
https://www.tp-link.com/support/faq/1615/