You will need to convert your UE smart Radio to a Squeezebox Radio which is done via a menu item on the Radio. Then you need to run your own server running some software called Logitech Media Server. The best device to use for this is probably a Raspbeery Pi. The best thing to do is join the forum in the link and ask for advice there, lots of people are in your position since Spotify made the old Plugin obsolete.
I really dislike that Spotify is gone, and I'm not geeky enough to embark on the Spotty solution, I think. But, whie biting the dust, I found a half-baked "solution" for the rest of us: Plug in a Chromecast Audio in the aux input. Then you will be able to play all sorts of Chromecast stuff, like Spotify, TuneIn Radio and much more. In addition, Chromecast Audio is also ready for muliroom (which functioned rather poorly on Squeezebox Radio when syncing Spotify). They're rather cheap. It's only 25 pounds plus 2 pounds for p&p here: https://www.mymemory.co.uk/google-chromecast-audio.html?source=Webgains
Iv used a chromecast fob on a Squeezebox Radio before. my only issue is that the input on the Squeezbox is not adjustable to line level... which causes for some volume flakiness... when this would be easily remidied by Logitech with a patch... to either add the option or set the input jack to line in level by default..
After reading three pages of complaints, I still can't believe that my streaming device has been deprived of its most important function without any permission from me and not to mention any communication about it. I am so, so disappointed with Spotify (as this change comes from their end). This is the type of attitude that really makes you dislike a service and start considering cancelling it. Shame on you Spotify