I am using a F@ST 3864AC modem with Optus NBN.
Wifi and Wired Ethernet to Apple TV works fine.
Same cable to an Xbox 1 or Sony Bravia TV no luck.
Spent hours playing with IP addresses etc but not sure what I am doing 😟
On you Xbox and Sony TV, there should be an option for automatic IP / DHCP or something similar. Select that rather giving a specific ip address.
If that does not work, can you confirm what IP address the 3864 and as well as the Apple TV.