EX3110 says incorrect password!

I've bad one of these before and it worked first time, no problems so when I went and botu this one, I expected the same! 

So I first tried the WPS connection. All seemed to work fine, but when I try to connect to the _EXT wifi, it says incorrect password. Now, believe me - It is 100% the right password. 

Next, I factory reset and tried to connect using the Netgear assistant. Again, went through all the steps with no issue, but this time when I try to connect to the _EXT wifi, the assistant says there's no internet connection and then just re-runs through all the steps. 

I'm literally tearing my hair out wih this thing! I've done both connection options multiple times. 

HELP! 

I've just done WP again, and the three lights (router, WPS, power) are all green. Only the device one is off. But when I try and connect (and I've tried both my HP laptop and our iMac) it just says incorrect password.