Spotify radio is not available on any partner devices, since it is missing from the Spotify API. There are numerous topics about bringing that functionality into the API, mainly about Sonos but any chance would effect all of the partner services, topics such as this one on the Ideas Exchange.
The Remote Player application and the software which implements the Spotify API on partner devices are not made by Spotify, but by the partner company instead.
I also started encountering the same problem after I upgraded my Denon to the latest firmware. I am still looking to solve this problem. Otherwise, I'll just probably cancel my Spotify premium subscription.