Authentitication Problem N300 EX2700

 I've set it up fine, connected it to the router and can see the wi-fi with the EXT at the end but when I try to connect to it on my mobile I get Authentication Problem. I've tried the same, trying to connect the Firestick, another laptop and I get the same message. 

I can connect on WiFi normally to the wi-fi without the EXT fine, albeit with a poor signal! I've logged on to the router itself and the security mode is WPA-PSK and WPA2 with new devices allowed. What am I doing wrong? I've tried numerous times to set it up and the signal is great on the extender I just can't connect to it!

 

I've also tried renaming the SSID on the Plusnet Router to something different, as well as changing the password and it still isn't working.