One of the factors that you need to consider when dealing with network-related issues like "not registered on network" is the IMEI number of your device. In some cases, a simple restoration of the IMEI number can get rid of the transpiring network error on your phone.
This guide will help you find and verify the IMEI number of your Samsung Galaxy S4. Follow these steps, whenever you're ready.
1. To get started, tap on Apps from the Home screen.
2. Next, tap on Settings.
3. Under Settings, tap on the More icon located on the upper-right corner of the screen, next to Accounts.
4. You will then be prompted with more options to view/configure. Scroll down to and tap on About Device.
5. Under About Device, scroll to and tap on Status. Doing so will show current status of your phone's network, battery, etc.
6. After you've tapped on Status, you will see the IMEI number shown in the Status screen, along with other phone details including the phone number, IMEI SV, IP address, Wi-FI MAC address, Bluetooth address, Serial number, etc.
Another way to access and view the IMEI number of your Samsung Galaxy S4 is by means of a shortcut code. Simply dial *#06# on your phone dialer and you will directly be routed to the device IMEI number.