If you are facing null IMEI and Not register on network issue on Samsung Galaxy S3 then you must try this method. First your device will be running Android 4.3 XXUGMK6. Now all you have to do is flash only 2 files through recovery. I’ve tried this and it has worked.
XXUGMK6 Modem.zip Download
XXUGMK6 Kernel.zip Download
Samsung Galaxy S3 I9300 IMEI Restore Network Registered Correction
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