You can download My ATT app. It allows you to login to your account, pay bill, make changes on a postpaid account.
Postpaid customers can login online with a user name, password and then passcode or PIN number. In the past, the last 4 of social security was the default PIN. ATT asked all customers to choose a new 4 digit number starting this past summer, following numerous incidents of account hacking. This is possibly when they took away the access to the account in settings on phones and tablets. If the iPad isn't locked, anyone could access that information.