Multicast with 2 Networks

I have a SG135 with UTM Home and a Fritzbox 6591.

My Network is a followed:
ETH0: WAN with IP 172.10.0.2 (Fritzbox 172.10.0.1 / Additional IP 192.168.3.1 with DHCP on UTM for WLAN over Fritzbox)
ETH1-7: internal Network with 192.168.4.1 and DHCP
Pi-Hole with IP 192.168.4.3, working as DNS Server for internal network and WLAN, also there is Avahi installed with mDNS reflector enabled

All is working fine, but I can´t get it to work, that via Multicast I can connect from my mobile (192.168.3.102) to my AVR (192.168.4.6) with SpotifyConnect or my TV (192.168.4.5) for Netflix.. when I put both devices from LAN to WLAN it works, but I would like to stay with the cable-connection on both devices.

I tried Multicast PIM on UTM, but I always get a neighbour not found in Live-Log, no matter what I try. 

Can somebody tell me, how to setup Multicast PIM to get it working like I want it to? Technically it should be no problem to realize I think, right?

My Multicast PIM is actually like this:
Interfaces:
one for internal on br0 (eth1-7)
one for WLAN on eth0

RP Router:
is set to the PI, so IP 192.168.4.3 and Group prefixes 224.0.0.0/4 and 224.1.1.0/24

then I have to routes:
external to Gateway (Pi)
internal to Gateway (Pi)

but I also tried interface-routes, also without success.