Hi
Sounds like typical Netgear, a product released into the UK which doesn't support fully FTTC as used here.
I'm very surprised it isn't supported, as it is a function of the Broadcom chipset and enabled by the driver, unless of course they need to pay extra for this I can't understand why they have left it off.
Do we know what the VDSL driver version is? It should be a simple case of a firmware update to enabled it, unless of course they are planning on releasing a D6400 Version 2 with full support leaving early adopters with a big black elephant of a plastic box in the room, which wouldn't be the first time they've done that. Still at least they managed to ship it with a nice illuminated logo which works out the box.
Regards
Phil