Starting today (August 18th, 2021), both the full, ad-free iOS app and the full, ad-free Android app are FREE!
Free and ad-free, full iOS app:
https://apps.apple.com/us/app/seterra-geography-full-version/id1093460065
Free and ad-free, full Android app:
https://play.google.com/store/apps/details?id=com.seterra.