Apps must be updated by the respective App Developer - not Apple - to maintain compatibility with newer versions of iOS/iPadOS.
Some App Developers abandon support for their Apps entirely, or cease to support older versions of their App(s) as newer versions of the Operating System are released. As described, the App with which you a having difficulty appears to have been fully abandoned by the Developer.
Either way, you will need to seek support from the App Developer - as only they will be able to help you with your issue:
How to contact an app developer - Apple Support