@StCath Hi, my advise will be, In Omada V6 Controller, there's mDNS relay toggle at :Configuration>Network Config>General Settings>mDNS.

Create a rule where device type is AP (or Gateway is u're using Omada gateway), bonjour service section you need to quickly browse through predefined service, if your VOIP mdns service is not in there, manually add a service with the mdns service address. (For example, google chromecast use  _googlecast._tcp.local)

Wireshark can sniff mdns packet from your voip phone by using a laptop, look for xxxx.tcp.local.

As for the multicast, everything through multicast my advise will be, make use of igmp snooping, you can ensure you igmp snooping config is correct or otherwise, but streaming a video via udp at one end, and use another pc to receive it at another end by using VLC media player or ffmpeg. It's very easy to do it with the help of copilot, chatgpt etc. 

Hope it helps, love from Malaysia.