Android TV - Not Supported (?)
As I have recently found out the streaming app does not support Android TV for several reasons. First being that most Android TV boxes are rooted. This causes other streaming apps to break as well. Netflix has the same problem with my rooted device. Xfinity dances around the subject just like Mediacom. Even though your device supports DRM, the fact you can take the finialized output and redirect it to another device, or even record it outright (not saying you are going to do that) Xfinity, like other streaming providers, simply looks at a few items on your device and says "Nope".
There are ways around this, that because I'm writing this on Xfinity's site I am unable to post here, but simple apps could help with this problem. It all boils down to DRM and media rights. Xfinity might, in theory, be willing to show the content, but original providers, like HBO, and others, may want tighter restrictions. Restrictions that make devices like Android TV boxes a NO GO. Especially with the Kodi lawsuits and EU restrictions (even worse than USA restrictions).
About the only way you can get the playback to work correctly is to remove root, Kodi, and other items that permit casting or otherwise would allow recording. While you may hunt till you drop for all the 'bad items' there are a few versions of Android TV OS that have all that built into the kernal or generally in the OS itself and cannot be removed.
If you want to stream you could use Chrome Browser to do it. That or use the apps for the specific service providers (USA, HBO, etc.). They usually ask for your Xfinity info and will allow it to run on your device. At that point Xfinity does not care as it becomes the provider's problem. If you can play via the browser that would be the best way as the broswer sandboxes the stream and the DRM information and controls how it can be redirected.
I know this is not much of an answer but the best we Android TV users can hope for.
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