Open up the dialer and enter
*#06# to check your phone’s IMEI. If IMEI is Null or Network is not registered (Network detected but you cant call or browse) then this guide is for you. * In the dialer , type-
*#197328640# or *#*#197328640#*#* * You will be prompted into the Command mode
* On the screen provided , Select Common
* Now Select FTM (Field Test Mode) ; FTM is for debugging.
* If (FTM) is on, you will have to turn this OFF
* As soon as FTM is off, your NULL or altered IMEI number will be restored * Without leaving the Command screen press the Menu key
* Select Key input and enter option 2. In this case you want to turn it off
* Wait for a minute , this will turn your FTM OFF
* Remove your battery and sim card for 2 minute
* Slot your battery inside you Samsung galaxy Phone ( do not place your sim card in)
* When the phone is booted, enter the command mode again dial *#197328640#
* Select the Debug Screen * After that, select phone control,
* Click on nas control
* Now, on the screen provided, click RRC(HSDPA)
* To Fix not registered on network or Null IMEI # , click RRC revision
* Now, click on the released version your phone is (here we will choose HSDPA )
* Final step to fix Null IMEI not registered on network for Samsung Galaxy is to turn off your phone then insert your sim or Insert your sim card then reboot your phone
Source: How to fix Null IMEI / Network not registered on Samsung Galaxy
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