IPTV and IGMP proxy: limit to a single port?

I have the TV7 IPTV service in Switzerland. I followed the instructions at https://www.init7.net/de/support/routerinfos/turris_omnia_igmp-proxy.pdf and all works fine streaming to an AppleTV or to VLC on a computer.

However, I have a second WiFi access point connected to one of the port of the Omnia (an old AirPort Extreme configured as a bridge to extend the network). Using IPTV makes connection to the WiFi from that access point impossible. I see the network, sometimes I connect, but there is no connectivity.

I know WiFi and IGMP proxy are not supposed to work well. Is there any way to limit the IPTV multicast to a single port on the omnia (where the appletv is hooked) and avoir that other WiFi access point completely? Any other step I could take to improve my setup?

Thanks in advance for any wisdom anyone can provide!