Good news! I've successfully removed your payment arrangement and we're good to go for creating a new one. Also, to avoid $10 Support fee. I'd be happy to help by sending the steps for payment arrangement self-services!
Here are the steps for you to be able to set up payment arrangement using the T-Mobile app:

1.) Open the app and tap BILL.
2.) Tap Set up a Payment Arrangement, Continue and view the Amount
3.) Tap Payment Method.

Please let me know once you are done so that I can double check it here in my end. Thank you :)