For future references, here's the step on how to set up Autopay from your app:

1. Log in to the T-Mobile app.
2. Select the MORE tab, select Profile.
3. Select Billing & Payments, then select AutoPay.
4. Select Add a card or Add a bank.
5. Enter the required payment method information and select Continue.
6. Select Agree and submit.

https://www.t-mobile.com/support/account/autopay