R6700v3 - can't connect via Nighthawk App

TL, DR: I have the same problem using the Nighthawk app as the other user: it returns a message saying the router is unreachable when I try to log in.

The overlong version: A little over a year ago, I bought a R8000-100NAR Nighthawk X6 AC3200 router to replace a Norton Core router that Norton discontinued/bricked.  I was never able to get my older HP printer/scanner to connect to the new router, although it worked fine with the Norton Core.  A few days ago, I picked the printer up and carried it to the room where my computer is.  I connected it with a cable and went through set-up with the HP printer set-up utility on my computer.  It failed, returning a report that it was probably due to MAC address filtering. 

However, when I tried to check the MAC settings on the router, I was unable to login.  When I set up the Nighthawk app, I set it up to log me in my reading my fingerprint.  However, after reading my fingerprint, it asks me to log in with a password.  After many failed attempts, I finally guessed the password I had used, only to have the app tell me that it can't run because my router is unreachable.  The router runs fine, everyone connects happily, but I cannot add this device to my network because I can no longer open the app to check and change settings. 

i looked at the HP link posted, but for me it opens here https://www.hp.com/gb-en/shop/list.aspx?fc_conn_wifi=1&sel=PRN.  Possibly because I am not in the UK.  It's really not a problem doing set-up, as it worked on the Norton Core, it's a problem with a default setting on the nighthawk router.  Possibly an "out of the box" problem, if that term refers to using a device with its default configuration.  Unfortunately, I can no long get into the router to change anything.