There are three option you can choose between one is via online website or by calling customer care or by mailing to their address with payment. For paying using their website that is by online method. First is to visit citi login page as universal card is issued by citi bank. Second is enter the id and password you got while registering on universal website. Third is click sign on option. Forth is select the payment selection of at&t universal card. Fill out details that are being asked. Such as name, account details. Then click on pay the bill and your bill is paid, its that simple!
Source: https://www.accountiod.com/att-universal-card-login/