To sideload apps on Fire TV and Fire TV Stick, follow these steps:
- Enable “Developer Options” by going to Settings > My Fire TV or Device & Options > About > Click the device name until a message says “You are now a developer.”
- Allow apps to be installed from outside the Appstore by enabling “Apps From Unknown Sources” at Settings > My Fire TV or Device & Options > Developer Options.
- Use the “Send Files to TV” app on your Fire TV and phone to transfer the APK file and install it.
Sideloading is the process of installing apps without the Appstore. We’ll manually download the APK and install it ourselves. There are apps made for Android TV and Google TV that aren’t available in the Amazon Appstore, and some Android apps work fine on a TV despite not being designed for it. Sideloading allows you to use them on a Fire TV.
Read more at: https://www.howtogeek.com