AC750 DUAL BAND WIFI RANGE EXTENDER

I've been and purchased the above wifi extender, in the hope it would help with the poor wifi we get in the area.

Plugged it in, pressed the WPS, got all the lights on green .... but can't connect ANY devices to the extender.

The guide says to use the current wifi's password but its not recognised. I've used every code I can find but no luck.

The green device light on the plug lights up when attempting to make a connection, but then goes off again when password is not recognised. Any advice please? I've already been back to the shop and replaced saying the first must of been faulty! Thanks