Can't change my payment method

I can't update the payment method in Apple Pay. My card details haven't changed but they did decline once last month. Now it won't let me update the card details, delete the card, or add the card again. I don't want to add a different card because this is the one I use for my bills. The update card details page doesn't let me change any of the numbers or other details. If I try to remove it, I get the message that there are subscriptions attached, which I know, so I can't delete it. If I try to add it again, it says it's already added. It's a frustrating circle that is stopping me from downloading apps on my phone.

Please don't suggest any of the "help" answers because I've tried it all. What can I do to solve this issue?