Hi @Anonymous User
If you Google the problem, there is a couple of websites that gives possible pin pointers to what the cause maybe and how to possibly fix it.
Here's the first website from my quick search.
https://theconchtech.com/streaming-video/amazon-firestick/why-does-amazon-fire-say-authentication-problemsolutions/
I use an Amazon Fire Stick and Amazon Fire Tablet with no problems connecting to the NOW Hub 2 Router.
There has been a couple of reports from Fire TV and Android TV users that they couldn't connect to their NOW Router and disabling IPV6 fixed this for them.
Also when typing in the NOW wifi password make sure everything is in Caps and don't get mixed up with O's and zeros.
