You cannot sign into the pre-installed Youtube app. To sign into Youtube you need to do two things:
1. Install and sign into Google Play Store. The sign in for Youtube (and other Google apps) is managed by Play Store. You can install and sign into Play Store very easily by going Android Apps - Play Store. It will require a reboot before sign in.
2. Install one of the Youtube apps from Play Store which allow sign in.
Dune have disabled Play Store and Google accounts by default because it runs in the background and in some rare cases can interrupt media playback. (although Play Store is installed on every other box we sell, so you should see no problems).
The pre-installed Youtube app is Youtube for Android TV, which is not available in Play Store. The only difference I can see between it and official Youtube for Android TV is the inability to sign in. You can install (sideload) official Youtube for Android TV to get a Youtube app where sign in is possible and there is good controllability via the Dune remote. Download from here: https://www.apkmirror.com/apk/google-inc/youtube-for-android-tv-android-tv/ (arm-v7a version). Copy to USB stick, insert stick into Dune, open in file browser (you will be prompted to do this), select file, select Install.
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