mutsu Hi,
I have same experience with this device long ago, very hard to repair IMEI, rewrite success but imei remain null.
But finally I have success and will make a guide for you to follow.
At first, you have to erase modemst1, modemst2, and fsg in EDL Mode.
We need special modem firmware, you can download it here(Non-HLOS + qcn file), and re-write it via EDL too.
Power on device, activate usb debug in setting, and root device using King Root apps.
After root, go to Service tab, and Click on Read button to enable diag port.
Select diag port in com port, fill imei number, select QCN file and do write QCN.
I just got success with above methode, I hope you too,
So GudLuck with you.
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