Xiaomi
Turn SIM PIN Lock On or Off auf Xiaomi
The SIM PIN is a security layer built into the SIM card itself, separate from your phone's screen lock. It stops anyone who pops the SIM into another device from using it, but it also means you enter a code before you even reach your pattern or password screen.
On MIUI, this setting lives under account security, not display or lock screen options, which is why many people struggle to find it. Go to Tap on Settings ➔ Tap on Passwords & security ➔ Tap on Privacy. If you run dual SIM, MIUI asks you to Choose a SIM card (Dual SIM) first, since each card has its own independent lock.
From there, use Turn On/Off Lock SIM card to flip the setting. MIUI always asks you to confirm with your current PIN before applying the change, via Enter the current PIN and tap OK. Get the PIN wrong too many times and the card locks itself and demands the PUK code from your carrier, so only proceed if you are sure of the current PIN.
Turning the lock off removes one prompt on every restart, but it also means the SIM works in any phone without a code. If your device gets lost or stolen, the SIM itself stays fully usable until you block it through your carrier. Keep that trade-off in mind before disabling it permanently.