For anyone who owns an Asustor NAS and wants to manage it via the AI Master app, Asustor now offers the app in native Android. However, it is not currently offered in F-Droid, Aurora Store or APKPure app stores.
It can be downloaded here: https://www.asustor.com/service/downloads?product_id=69#android_app
If you need instructions on how to sideload an app, this article will give you simple instructions: How to Sideload Apps on Android
I had no problem downloading and sideloading the app, which fired right up, found my NAS and enabled me to login immediately.
Hope this is helpful information to the few who might use an Asustor NAS.