The Plex for Android app is not a DLNA client. It cannot see DLNA devices lie your Marantz. You need to use another DLNA client for Android to stream to your Marantz. If your Marantz supports Chromecast, you can use the Plex app with that, but if you’re not seeing it, then it likely does not support Chromecast. Our Plex for iOS also is not a DLNA client, but it can Airplay so it can work that way.