EAX20 (AX1800) will not accept my TalkTalk router password and mywifiext issues

Recently I have been having connection issues with my 4 month old extender with error with it saying mywifiext  is not trusted and devices will not work on the extender. I decided to reset the router and start again. But now when using the nighthawk app to set up the extender it will not recognise my router password and definitely typed in correctly. Any thoughts?