UPnP device discovery across VLANs

[alto]

I’m trying to set up a streamer on a separate IoT network in my LAN so that the Symfonium android app can cast via UPnP to that device from a trusted network, keeping the streamer isolated from other devices but still accessible for casting.

Symfonium is the casting app, with a Wiim Pro streamer and a subsonic media server source. The media server and Symfonium phone is on VLAN “Trusted”, and the Wiim Pro is in another VLAN “IoT”.

The problem I’m having is that both the android apps for Spotify and Wiim can reach the steamer across my VLANs (I have set up mDNS multicast across the networks), however Symfonium is unable to find the streamer as a UPnP device if the phone is connected to the trusted network. If I connect the phone Wi-Fi to the IoT network, then Symfonium immediately finds the streamer and can cast audio.

I'm wondering what network configuration is needed, or even possible, in order to have UPnP traverse across VLANs?


Additional information:

I have temporarily enabled full network access between the VLANs to minimize any other sources of error. I also have firewall rules opening traffic to 224.0.0.251:5353 both ways between the VLANs to support mDNS multicast, along with a mDNS-repeater between the networks.

Summary

    Spotify and Wiim apps can discover the streamer device across VLANs
    Symfonium can not discover the streamer across VLANs
    Symfonium can discover the streamer if connected to the same VLAN
    Firewall rules are in place to support cross-network discovey with mDNS
    All other traffic between the two VLANs is currently open in order to mitigate other sources of error.