There is no Payment Method field in the Bank Register window

The payment method and details used are the ones you assigned to the customer in the Payment Details view - Customer of the Card Information window.

If there is no default payment method assigned to the selected customer, no payment method is assigned to the transaction.

Click the Split button to open the source window to change the payment method or details if they differ from the default information or to assign a payment method and details if there is no default.