The No ads and PRO purchases are made in the app itself, through the app store of your device: Google Play on Android and the App Store on iOS. Both are one-time purchases, with no subscription. The store shows the price in your local currency before you confirm and charges it to a payment method from your Google or Apple account.
Restoring purchases
A purchase is tied to the store account it was made with. On a new device or after reinstalling the app, sign in to the same Google or Apple account and tap "Restore purchases" in the app.
A purchase does not carry over between platforms: if you use the app on both iOS and Android, each platform needs its own purchase.
Refunds
Refunds are handled by the store under its own rules. You can request one on Google Play or through Apple.
Payment problems
If the payment went through but the purchase did not show up, first tap "Restore purchases" in the app. If that does not help, write to [email protected] and attach the receipt from the store.