Unable to change the encryption key on Nighthawk AC1900 Model R7300 DST

I need help, as I cannot get the encryption key to change/update OR maybe Im not understanding exactly how it works (yes, I have hit the apply button).  I have reset router to factory NUMEROUS times and cannot get it to change, although I can change the router name.

When we first got the router, Best Buy set it up with zero instructions.  They had set up the adapter as well, but we have never used and have NO idea what the password could be.  I have also reset the adapter to factory default.  Do i need to connect the adapter again to change the encryption key for the router even though we are not going to be using the adapter?  And is it true that if someone has the encryption key, they can use an adapter they have to get into my router and settings?!

Thanks! :)