Yeah I think you have the same problem I described. When the BT box starts viewing BT sports it will cause the IPTV multicast packets to go to your unmanaged switch and then from there to your Sky Q box and then to your wifi network. Constant packets can easily cause packet collisions on wifi networks specially on wifi meshes which use "point to point wifi bridges", which is what Sky does. The better wifi meshes, such as Ubiquiti Unifi, use wired APs and don't use point to point wifi bridges since these add latency and can saturate the wifi radio channels. The best solution to your problem will be to connect the BT box directly to your Asus DSL-AC68U router. Make sure IGMP Snooping/Proxy is enabled on the Asus and your multicast issues will be gone. If you can't do that the next best thing will be to create a separate network path for the BT box. You could have a separate ethernet cable from the Asus to a separate new switch (can be unmanaged) which then connects the BT box. A variant of this will be to replace the unmanaged switch with a switch that allows VLANs so that you can isolate the traffic between the BT box and the Asus router, but again you need another network cable between where your new switch is and the Asus router is. A third option could be to use a pair of Powerline adapters to connect the BT box and your Asus router directly via a separate network path. Hope it heps!
Thanks for that.
Unfortunately running additional cables is currently not an option , nor are powerline adapters as his mains wiring seems to have issues when we've tried this previously.
With a managed switch is it really necessary to again have that on it's own ethernet cable to the router?
I think a managed switch should work as long as the managed switch supports IGMP Snooping/Proxy and that's enabled. See this article for a sample topology:
In the first image your Asus router will be the "gateway router" on the TP-Link FAQ article. Your Asus router will do IGMP Snooping/Proxy and send all the multicast traffic to the switch via a single connection. This is not ideal since you will be mixing IPTV traffic with regular IP traffic but should be fine. Then the "smart" switch will again do IGMP Snooping/Proxy and only send multicast packets to the BT box port, as it is the only one requesting them. That will prevent the other boxes on your network from being "spammed" with the multicast traffic.