I’m not familiar with ‘Radio Tunes’ as a service provider. Have you contacted them to suggest they become a service provider in Sonos, per the instructions found on the Sonos partners page? If they provide an internet link, you could also set them up individually as a ‘station’, by following the instructions in the setting up an internet radio station FAQ. 

From the radiotunes website:

Hardware player integration requires a RadioTunes Premium Subscription

You can use any hardware player that accepts custom URLs. Here are some of the most common hardware players used by our Premium listeners: Sonos, Denon, Pioneer, or any players that support vTuner or TuneIn.

Must be region specific, then, as it doesn’t show up as an option in my US Sonos controller. But that’s as a ‘separate’, I didn’t look as a TuneIn access point in my Sonos. But the quote suggests it is a separate streaming source, so worth looking for in the ‘add a service’ menu.