What model was the Netgear you previously had? If it wasn't VLAN aware (unmanaged) and the TV equipment is using VLAN tags, the unmanaged switch will happily pass the traffic. The unconfigured ports on the 6450 just default to untagged VLAN 1; if the Freebox packets are VLAN tagged, the switch is either discarding the VLAN tagged packets or honoring them, but has no tagged ports to send them to. Was the Netgear that the TV system was plugged into shared by the rest of your network? Have you also confirmed that the Freebox connected to the TV is pulling the correct IP address to work?
I could be very wrong about what the 6450 is doing though. As this is supplied by your ISP, if their support is worth anything, they might be able to give you some guidance as well.
You can use
diagrams.net to draw quick diagrams of how it was before and how it is now.