RM Plugin app was designed to work only when it is installed directly from Google Play Store. This is to prevent any form of piracy or virus infection when installed from 3rd party sources. Side-loading from APK file to FireTV will result in 7-days trial, which will eventually expire.
We have tried to submit our app to Amazon AppStore to get it officially supported by Amazon devices, however we were faced with legal challenges from Amazon team. Therefore, we will stop our attempt to get it on Amazon AppStore.
We would advise you to purchase another Android device with Google Play Store for this purpose or root your FireTV to get Google Play Store on it.