Option 1: Use Unlock Code (If Available)
If your router allows manual unlock via code (some versions do):
Insert a SIM card from another network (not the original network).
Access the router’s admin page:
Usually via browser: 192.168.8.1
Login using default credentials:
Username: admin
Password: admin (or check the sticker under your router)
You may be prompted to enter an unlock code.
You can buy or generate unlock codes online based on your IMEI.
To find your IMEI:
Check under the router or on the box.
Or dial *#06# if available via interface.

You can try getting an unlock code from sites like:
dc-unlocker.com (ρáíd)
routerunlock.com (ρáíd)
Option 2: Flash Openline Firmware (Advanced)
If your router doesn’t accept unlock codes, you may flash openline firmware. This is risky and should be done only if you're confident.
What You Need:
USB to LAN adapter (if needed)
Huawei B312-939 firmware tool (specific to model)
Correct openline firmware file
Access to a Windows PC
General Process:
Download the firmware tool and openline firmware for B312-939.
Connect router to PC via LAN.
Run the firmware flashing tool.
Put the router in Download Mode (usually involves holding the reset or WPS button while powering on).
Flash the openline firmware.
Wait for router to reboot.