When you’re using App Pay or "Tap to Pay", (Fob attached to phone), the Button Pay button on the side of the Fob doesn’t work.
If you Fob is attached, you’ll need to Tap your credit card inside the App to pay. But the beauty of Tap to Pay is you can use any payment card in your Wallet menu to transmit your payment.
To use the button on the Fob, it needs to be detached from your phone. We call it Button Pay and it uses uses your default payment card. If you want to use a different card with Button Pay, you can change the default card while your Fob is attached. Then detach, and Button Pay by pressing the button.