How i add my payment method
How to uc purchace by sim card
iPhone 12 Pro Max, iOS 14
Question marked as
You can change your payment information following the steps at Change, add, or remove Apple ID payment methods - Apple Support
Being able to use your mobile network / sim-card as a payment method is only available in certain regions and specific carriers too. If you don't have the option, it is sadly not available to you at this time.