rusEFI Android App is part of rusEFI Online suite. The primary job of rusEFI Android app is to provide rusEFI ECU with remote tuning capability.
One day we will publish rusEFI Android at the google store but for now you would have to install it using rusEFI-release.apk
You would have to agree installing application not from Play Store and you would probably have to disable Google Play Protect.
Q: How to disable "Google Play Protect"?
A: On Android >= 8 Settings => Apps => Google Play Store => Notifications => High priority => Turn OFF
On Android <= 7 Settings => Notifications => Google Play Store => Block all