The ad-free version is actually a completely separate app in the store, rather than an upgrade inside the free app.
If you are still seeing ads, you are likely still opening the free version. To fix this, just download the paid version from your device's App Store or Google Play Store. Once it's installed, we recommend deleting the free version to avoid any mix-ups!
Note: Your purchase is tied to your Apple ID or Google Play account. If you switch to a device using a different store account (like moving from iPhone to Android), your purchase cannot be transferred.
In case you have a new device and cannot complete access to the ad-free version, we recommend you visit the article: Why am I being asked to pay if I have already purchased the ad-free version?