How to restore IMEI and get back network functionality?
If you also have "No Baseband" in the about screen, You can try to flash only the modem and see if it help.

e.g With Samsung devices, I usually use Odin with the ROM from SamMobile, 
I extract the modem file from the archive and put into a new .tar file, and then flash the new file with Odin.

On Linux i can use Heimdall which allow my to directly flash a specific region like the modem.