App Store payment method issue after changing Apple Account region

I faced an issue when I changed my Apple Account region from Serbia to Slovenia using Slovenian mobile phone number as a payment method. After that I linked Apple Pay as payment method and thought from now on every purchase in App Store will be charged from Apple Pay. But when I did a purchase it only attempted to charge from mobile phone account. When I try to change my region back and link the card I used before it doesn’t let me even though I was using this card before as a payment method. I believe that this issue is related to insufficient funds on my Slovenian mobile number. I’m physically in Serbia right now so I need to change my region

[Re-Titled by Moderator]