For most REALME C12 users, (Power off → Vol Up + Power → Select Fastboot from Recovery) is the most reliable. Once you’re in Fastboot, you can unlock the bootloader with the command fastboot oem unlock — but remember, that will wipe all your data.
Note: If the phone boots up normally to your lock screen, simply turn it off again and try the timing. It requires a simultaneous press, not one after the other. How to Open Fastboot Mode on REALME C12
Use the Volume Down button to navigate the menu and select “Fastboot Mode” . Press the Power button to confirm. For most REALME C12 users, (Power off →