Registering with Serial - EX2700 - and WPS every time

Im trying to extend the router to my bedroom however I have to go to settings on my phone (Sony Xperia Z3), Wifi, Settings then WPS push button evertime to connect. If I try normally it comes up with Authentication problem (tried the password many a time - works on the main router but not on the N300). And when trying to install my Chromecast to the tv in my room there's no sign of it at all!

So after all that i tried to register with Netgear however it states that I may have the wrong serial number and cant submit - ive tried the MAC, serial number and WPS PN too.

Much appreciated for any help to the donkey here!!

Cheers