Thanks. Rather than being based on AssetUPNP as I thought and mentioned above, other googling suggests it actually has LMS/Lyrion at its core. As such, it wouldn't be directly accessible via the WiiM Home app unless it had enabled the obsolescent UPNP/DLNA interface plugin - if it allows that degree of customisation.
If you're using the Material Skin, doesn't the Pro Plus appear as a player there? Check in the WiiM app under its device settings that it has Squeezelite enabled.
Alternatively, there would appear to be a setting which could make your Zen visible as a UPNP server in which case you should be able to browse i from the WiiM Home app - see SYSTEM 🞂 SETTINGS 🞂 INTEGRATIONS 🞂 UPnP page in the Innuos Sense app.