EX6120 Can't connect extender to my devices

Hi newly bought AC1200 Dual Band WiFi Range Extender (EX6120) through Amazon. Had good reviews. 

Followed the Quick Start guide to set up Extender Connedcted to Router no problem. (Router = technicolor TG588v v2)

When attemping to link a mobile phone and Laptop PC to the extender - I see the required two extentions _2GEXT & _5GEXT I get a "Could not Connect to WI-FI Network error followed by a Check password and try again message for all devices I am attempting to access the extender. I know the Wireless Key/Pasword I am using as instructed is correct as it works for my devices when accessing the router. 

What am I doing wrong? What password shoudl I be using? It is very fustrating.

I am based in Cardiff UK if country can be an issue. 

THank you in advance for your support.