Originally Posted by Leon
I always make EFS (IMEI) backup and it never helps me anyway when its break during flashing custom ROMs. What helps me usually is flashing modem and bootloader files.
That is indeed an option, but if you have not backed up EFS then a Nand Erase is the only recovery option.
Lost IMEI on my S9+ (SM-G965F).
Searched for solution for hours.
Eventually ended up with Nand Erase option checked in ODIN (v3.14.4).