Nimmiotheit93 I suppose you talk about the Swisscom TV box being connected to the router.
It is also possible to use an external router, but here the Enable IGMP Multicast Proxy must (!) be activated, otherwise it won’t work. IGMP Snooping would be good, too, but isn’t a must. More info about the usage of external routers you may find here:
Connecting external routers to the network - Help | Swisscom