This problem may be caused by the following situations. Please perform troubleshooting based on your actual conditions:

There are two cases where the PIN code appears after power off and reboot

1) One is the password lock of the SIM card, which requires a 4-digit password  The SIM card will be locked if you input the wrong password 3 consecutive times, and then you need to call the operator or use the PUK code on the card box to unlock the card.              

2) The second is the unlock password, which should consist of 6 digits. If you forget the password, hold down the volume button and the Power button at the same time to enter the recovery mode for factory reset, or contact customer service for version upgrade.