John, that indicates your networking setup, at lest from your Mac/iPhone connections, is working as it should, as are your UPnP Servers, and your MacOS security settings. VLC is a well trusted application for testing multicast.
I’m not sure how Mosaic ACTUS operates exactly, but with the standard Mosaic, similar challenges with discovering UPnP Servers have been noted elsewhere.
The following recent thread is an example, and I do recommend you read it in full;
One key aspect to note is the point I raised about how the UPnP Servers are actually initially discovered on the network, by the dCS hardware, not actually Mosaic (on your Mac/iPhone);
In any case, my suggestion is to let @Phil assist you in getting this resolved.
Sorry Pete, that’s incorrect. Every UPnP Control-Point uses the same method to discover UPnP Servers (via SSDP).
As I mentioned in the past, it’s other applications that use different multicast techniques for discovery, for example, how Roon discovers its Servers is different from how the Qobuz App discovers Qobuz Connect capable hardware, versus how Mosaic itself discovers dCS equipment on the network etc. They all rely on multicast, but differently.
Things get slightly more complicated with Mosaic because in some sense it’s not a pure-play Control-Point, it’s actually a hybrid graphical front-end to everything that the Mosaic Processor does, ie. the dCS network connected hardware (and firmware).