You can grab the file for Google app for Android TV v3.0 (Arm) from APK Mirror and sideload it on your Android TV. Source: +Artem Russakovskii ...

Sometimes, not every Android app will be available on the Google Play Store in your region, which means you'll need to install the APK file instead.

To do so, first, sideload Aptoide APK on your Android TV (be it Mi Box or any other Android TV) as mentioned above. Once Aptoide is installed, launch it and look for Prime Video in it.