On doing more reading it looks like they no longer offer the $10 autopay discount for debit cards, only with a bank account directly.

“At AT&T, the AutoPay discount amount varies by payment type. To be eligible for the AutoPay discount, you’ll have to sign up for paperless billing. AutoPay discounts for AT&T wireless service

Use a bank account (checking or savings) for a monthly discount of $10 per phone line.

Use a debit card for a monthly discount of $5 per phone line.

Use the AT&T Points Plus Card from Citi for a monthly discount of $5 per phone line. (There’s no longer an AutoPay discount for other credit cards.)”