@Richard_Burford I’m using multicast UDP just fine in my own production Sequoia app (non-MAS). Works fine for my customers too. If you have iOS in the mix, you’ll definitely need the entitlement though as IIRC things were a bit sketch until that happened (e.g. it would haphazardly work in development but not work at all in production). If it helps, see the findings I shared here:
iOS Multicast Setup & Best Known Methods
Otherwise, on the macOS side of things, I don’t think I had to do anything special other than accept the “Do you want to allow XYZ to access your local network?” dialog.
In my experience this should reappear if you delete all copies of your app that uses the same bundle ID on your Mac. Otherwise you can theoretically try and zap the permissions with tccutil from the Terminal, but the last time I tried under Sequoia I couldn’t get it to work (but I’ve used in many years ago just fine). Of course you can also try and fiddle with:
System Settings » Privacy & Security » Local Network » Switch for your app
Note that if you have multiple versions of your app on disk, you’ll have multiple listings. In this scenario, I couldn’t get the switches to work properly on my own system. At the time I was testing things, I think I was able to get the dialog to reappear by simply deleting the debug version of the app and recompiling from scratch.