A lot of times, phones can give the "Not registered on network" error, but this is not a reason to panic, since there are a few easy ways through which you can fix this.

Basically, this happens after unlocking your phone and trying to use a new network. The Service Provider you are trying to use is not yet notified of your IMEI number, and in consequence, you get the error on the pop-up screen Not Registered on Network.

You can find all the solutions here: https://www.unlockunit.com/help/knowledgebase/how-to-fix-not-registered-on-network/