"Unregistered NT Core" is a notification often seen when the app is unable to successfully connect to a Core. This can have a multitude of reasons, of which the 3 most common are explained here:

Empty Core battery

Please replace the Nt Core Battery with a fully loaded one.

See also:
I can’t switch on my Nt Core

My Nt Core immediately turns off again after I switch it on

Not logged in

You've been automatically logged out of the Nt App, due to your offline time expiring. Please log out and in again to renew your login authorization.

See also:

Why is the information icon red

You tried to connect the wrong or unregistered Nt Core

In this case, please contact your local distributor.