Billing

1. Credit or debit cards (American Express, Discover, Mastercard, Visa).
2. ACH transfers from a valid U.S. bank account.
3. Payments via the Xfinity app, which now accepts Apple Pay.
4. Payments through Xfinity Voice Remote by saying “Pay my bill.”
5. Mailing a check or cashier’s check drawn on a valid U.S. bank account.
6. Payments at Xfinity Retail Stores using self-service kiosks, which accept cash and traveler’s checks.
7. Money orders at Western Union Quick Collect locations.

These are the only payment methods accepted by Xfinity Mobile.

For more details see:

• https://www.xfinity.com/support/articles/how-to-pay-xfinity-mobile-bill
• https://www.xfinity.com/support/articles/pay-your-bill-echat

Does it state the resason your payment method is rejected? Also, have you verified the card is activated and checked the avialable funds on the giftcard?