In the Web interface there is Application → IGMP Snooping. I guess, you played with that already. Although I am in Germany too, I have no MagentaTV to test with. Did you ask the OpenWrt community whether this is compatible with MagentaTV? There exists a longer discussion…
By the way, do you really need the Brume-W as switch for for your setup box? If not, you could use the Brume-W as cascaded router and use the routing feature of your DSL modem instead.