Hello mate, I am in your current situation. But I already have the device.
Unfortunately at the moment I have not been able to configure the flint with sky wifi. On the contrary I have been able to configure the brume.
Please follow Flint GL-AX1800 - WAN6 gets all conf from DHCPv6 a part from IPv6 address - #4 by glsorre 
Hope they are gonna reply soon.