Payment method declined after changing regions

If you are trying to use your payment method from your previous country, you may not be able to. See: Payment methods that you can use with your Apple ID - Apple Support

"Make sure that the billing address for your payment method matches the billing address for your Apple ID. In some cases, you might need to call your bank or financial institution to authorize international or online purchases. You might also need to confirm that the mobile number associated with your payment method is correct."

You apparently can use a card issued in a different country but it has to be billed to you at an address that is in the country to which your iTunes and App Stores are set. This requirement makes it difficult to use a card from a different country.

The link provided in a previous post also talks about cards being declined, but that is largely in the context of Apple not being able to verify the legitimacy of the card itself, not its validity for use in a different country.

See the Apple Support article(s) below. Link to contact Apple Support in the article. 

Apple ID payment information - If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store*

Possible fixes.   Fix ‘Your Payment Method Was Declined’ Error In App Store.  -

                          Payment method declined in iTunes or the App Store.   -