Multicast (PIM-SM) in one wireless network for using "Be On Air App" (audio broadcast via WiFi)

Hi,

I hope someone can help me.

Setup:
- Sophos UTM SG330 (9.353-4)
- AP50 with 3 WiFi
- "Be On Air App": http://beonairapp.com

We want to use the App to translate different languages (streams) for our events with 200 clients (client app).
On the AP50 we set up a WiFi with no internet access. The test was successful with 1 client but with many it failed.
On the Website they say "it should support and has enabled UDP and Multicast".

So I looked into the "Multicast Routing (PIM-SM)" Feature on the UTM and read the article: https://www.sophos.com/en-us/support/knowledgebase/115488.aspx

It says that I have to configure 2 interfaces for the Multicast, but I just have one because we just use one interface for the WiFi.
Which interfaces to I have to put there?
For the RP-Router I dont know what to put there. The AP50?

Somebody have experience in this and can help me?

THANKS!