Do you have a backup of your EFS? A corrupt EFS corrupts your IMEI making you lose signals to mobile connectivity. Have you tried restoring from Kies doing a factory reset? If you have no backup of your EFS then you have no option but to send it in for repair.
Phone: Samsung Galaxy S4 I9505
Rom: AOSP GE 4.4.2 by Danvdh - Fast! | Der Kernel: KT Kernel
Tablet: Nexus 7 2013 32GB
Rom: OmniRom 4.4.2 | Der Kernel: Stock Omni
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