The ultrarendu is RoonReady. Have you tried setting the ultrarendu in this mode ? If so, it should appear in the Roon device list. I do not understand your need for a UPnP bridge here. Maybe I miss something.
You won’t get official help with this from Roon support as it’s not using Roon Ready or another Roon supported method as Squeezelite isn’t supported. This is strictly tinkering. You should contact Sonore as it’s their product for help or many users on Naim forum would be able to advice or @simon_pepper as he uses one might.
But have you enabled squeezebox support in Roon? As you need that to be on for it to have any chance of working.