I have connected the router behind the Swisscom Internetbox 2, Swisscom TV runs via the router and it works without problems (IGMP must be activated in the configuration).

Addition: According to the manual, IGMP Proxy can be enabled under IPTV, IGMP snooping under Wireless separately (or probably also for LAN, I haven't seen a screenshot there). Can someone with IPTV from Swisscom or Init7 confirm that it works, that would be super helpful, Merci!