For Miibox can you try this build:
Arm64v8a: https://artifacts.plex.tv/android-experimental/8.1.0.17362-1ddb3df9/PlexForAndroid-8.1.0.17362-1ddb3df9-Arm64v8aGooglePlayStdExo.apk
Armv7a: https://artifacts.plex.tv/android-experimental/8.1.0.17362-1ddb3df9/PlexForAndroid-8.1.0.17362-1ddb3df9-Armv7aGooglePlayStdExo.apk