new setup across vlans/subnets

[simbun]

Enabling inter vlan traffic by itself probably won't help with discovery as that's a broadcast thing. What you need to look into is repeating SSDP broadcast messages.
Having said that I tried this a few years ago now on Ubiquiti gear, and whilst I got mDNS working across subnets (Google smart speakers accessing a UPnP server and Sonos accessing online services via mDNS) I was never able to get pure UPnP working; but I don't know whether it was a limitation of Ubiquiti, UPnP or my patience though :-)