Connecting a second extender

Hi,

I'm trying to connect a 2nd wifi extender to my router. I have one upstairs which my son's amazon fire stick is wirelessly connecting to perfectly on both 2G and 5G EXT using the same password as the Router. However I'm trying to connect a second one in the games room downstairs as the wifi just isn't strong enough for gaming despite the router being in the hallway about 15 foot away from the console. I've gone through all the initial steps pressing WPS etc and the Router light on the extender is solid green and the desktop that I am using to send this query is connected to the extender via an ethernet cable so I know the extender is connected to the router. However when I try to connect either my phone or son's Nintendo Switch it just won't accept the password.

I must just add that my Partner has changed the Wifi name and password, the extenders where both working fine in exactly the same configuration before he changed it. I have reset both extenders prior to trying to connect to the newly named Wifi Connection but as I say one is working fine the other just won't connect wirelessly at all. 

I've tried using the set up wizard on the website but that just connects to the one upstairs. 

Any ideas? I'm tearing my hair out