You can set a SIM PIN code to prevent people from accessing data such as contacts stored on the SIM card without your permission, even if they physically have your phone and then steal the card ...

Now, after Face ID fails for the first time, or you swipe up from the bottom of the screen, you'll be able to enter your PIN code and unlock your phone or approve an Apple Pay transaction.

Select Enter Master Key and input the 8-digit number Nintendo provided. Now, you can set up a new 4-8 digit PIN code. Choose whichever number you want, and try not to forget it!