My card payments keep getting declined to buy something from an app and I have updated my details and tried with a different card but it still won’t t go through

Probably you have the wrong Country / Billing / Shipping Address in the Apple ID. This should be the same as the issuing Bank Country.

Log in to https://appleid.apple.com with your Apple ID credentials.

Scroll Down to the Payment and Shipping Section

Tap on Edit

You will be given both Billing and Shipping Addresses to edit.

Based on your country of residence and country you have selected under your Apple ID --> Payment methods that you can use with your Apple ID - Apple ...

Have you a Debit/Credit card that is issued to you that is valid internationally and is from the same Country / Region as your Apple ID.

See this for assistance, If your payment method is declined in the App Store or iTunes Store - Apple Support

I changed it but it’s still says my update card I’m really annoyed