@Stone1234 

Depends if your exchange supports DHCP on IPv6 on your Sky FTTP line or if it only supports DHCP iPv4 Option 61

You can only try, just make sure your internet settings on the M4 are set to use iPv6

How to Enable IPv6 on Deco M4

1. Ensure your smartphone is connected to your Deco Wi-Fi network.
2. Open the Deco app and tap More > Internet Connection > IPv6.
3. Toggle the IPv6 switch to On.
4. Select the IPv6 Internet Connection Type matching your ISP (Dynamic IP is the default).

Unfortunately if you only have IPv4 then it requires Option 61 which Deco Wi-Fi routers don't support 

  @Stone1234 

Hi,
If there is a fiber line between the ONT and the sky router, the Deco M4 can't be used instead of the sky router. 

If there is an ethernet cable between the ONT and the sky router, it's recommended to connect a PC directly to the ONT and check whether it has internet. If so, you can use the M4 instead of the sky router. 

If the PC has no internet connection when connecting to the ONT, please contact the ISP to confirm the configuration details, such as the internet connection type, username, password, and VLAN ID, then try to configure the main Deco. If the main Deco has no internet, it's recommended to connect the main Deco to the sky router for internet connection.

Best Regards