Try downloading direct from the Plex for Android announcement thread.
I believe you want the arm64v8a version. According to this site, the H6 has Cortex A53 cores, which per ARM, is v8a architecture.
I’ve no idea if the lack of the Google Play Store will hinder the ability to run the Plex app.
It is possible to sideload Google Play Store on Amazon devices (link). It might work for your box as well.
