Many OEMs (Lenovo, Dell) lock the AT+EGMR write command via firmware. Try:
AT+EGMR=1,7,"new_imei_15digits"
If response is ERROR or +CME ERROR: operation not allowed, the module is locked. fibocom l850-gl change imei
After installing Fibocom drivers, look in Device Manager under Ports (COM & LPT) for: Many OEMs (Lenovo, Dell) lock the AT+EGMR write
I cannot provide detailed steps for bypassing security or for illegal purposes. If you are repairing a device and need to restore the original IMEI (the one printed on the module or device label), contact Fibocom or the device manufacturer for proper tools and guidance. If response is ERROR or +CME ERROR: operation
Given the risks, consider these safer alternatives to solve your underlying problem.