To get the Unifi Security Gateway running with Swisscom / Wingo TV you'll need to create two configuration files on the controller and reprovision the USG.

The file config.properties needs to contain the line

config.system_cfg.1=switch.igmp.header_checking=false

The config.gateway.json file should contain the following

{
  "firewall": {
    "group": {
      "address-group": {
        "IPTV_Source": {
          "address": [
            "224.0.0.0/4",
            "239.0.0.0/8",
            "195.186.0.0/16"
          ],
          "description": "IPTV Source"
        },
        "SCTV2_Source": {
          "address": [
            "213.3.72.0/24"
          ],
          "description": "SCTV2 Multicast Source"
        }        
      }
    },
    "name": {
      "WAN_IN": {
        "rule": {
          "3100": {
            "action": "accept",
            "description": "allow IPTV Multicast UDP",
            "destination": {
              "group": {
                "address-group": "IPTV_Source"
              }
            },
            "log": "disable",
            "protocol": "udp",
            "source": {
              "group": {
                "address-group": "SCTV2_Source"
              }
            }
          },
          "3110": {
            "action": "accept",
            "description": "allow IGMP",
            "log": "disable",
            "protocol": "igmp"
          }
        }
      },
      "WAN_LOCAL": {
        "rule": {
          "3100": {
            "action": "accept",
            "description": "allow IPTV Multicast UDP",
            "destination": {
              "group": {
                "address-group": "IPTV_Source"
              }
            },
            "log": "disable",
            "protocol": "udp",
            "source": {
              "group": {
                "address-group": "SCTV2_Source"
              }
            }
          },
          "3110": {
            "action": "accept",
            "description": "allow IGMP",
            "log": "disable",
            "protocol": "igmp"
          }
        }
      }
    }
  },
  "protocols": {
    "igmp-proxy": {
      "interface": {
        "eth0.10": {
          "alt-subnet": [
            "0.0.0.0/0"
          ],
          "role": "upstream",
          "threshold": "1"
        },
        "eth1": {
          "alt-subnet": [
            "0.0.0.0/0"
          ],
          "role": "downstream",
          "threshold": "1"
        }
      }
    }
  }
}

On Linux these files need to be saved in /usr/lib/unifi/data/sites/default/

Please note the use of eth0.10 for the upstream interface, eth0 did not work with my USG.

After you added the files to the controller, log in to the webinterface and select the UniFi Security Gateway 3P. Don't restart it but reprovision it by clicking Provision under Config, Manage Device.