Adding Test Credit Card to Apple Pay

The first thing I would do is set my region to one of the supported ones for test and make sure I remove all payment from wallet and settings.

Then create a new sandbox account. The common item is mixing the production stream with sandbox stream and burning your account status in the sandbox.

Important: If you mistakenly use a sandbox tester account to sign in to a production environment, like iTunes, on your test device instead of your test environment, the sandbox account becomes invalid and can’t be used again. If this happens, create a new sandbox tester account with a new email address.

It’s unlikely your developer account would be valid on device when testing payments in a sandbox with sandbox banking details.