Network issues become very common on android phones and often users have to tolerate the issue of phones not registered on the network. If you are one of them and thinking about the causes of the issue then you should read below:
1. Reboot your phone, it will troubleshoot the issue and the glitches of the device.
2. Activate your airplane mode and then off it. Airplane mode forces the device to connect with the available network.
3. Take your SIM card out of the phone and check whether it is damaged or broken somewhere.