"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.
I can’t switch on my Nt Core
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.
You tried to connect the wrong or unregistered Nt Core
In this case, please contact your local distributor.