IIMEI Changed to 0049.. and 'not registered on network'

Dear,

I think that my question was raised many-time in this forum, and i really tried to read any thread similar to my issue, and i tried all solutions but no luck, and I am feeling that i am missing something, here my story :

Problems started with me when i bought a new battery for my phone ( i don't think that this cause the problem, because as my knowledge imei has nothing to do with battery), please correct me if i am wrong.

So, when i putted the new battery, I got this silly issue 'not registered on network', and then i knew that i lost my imei as well (it is dummy number like 0049... )
So what i did is i tried to flash the modem, i tried many version, but this did not resolve my problem.
Then i tried to install the firmware from the beginning and rested all data and wipe all data and cache, (here i think i missed to take backup of my efs files). does flashing a new firmware,reset or wipe data will remove the efs files?

Installing new firmware didn't work also.

So i tried to backup and restore efs files, but it did not work also.

Please can you help me resolving this issue, since now i am out of idea and i don't know what to do.
Thank you in advance