Why do you think that it is not IPv6 compatable? IPv6 is IPv6 no matter which country. While I also have a VR900 it is a:

Firmware Version:0.1.0 0.9.1 v0069.0 Build 160721 Rel.66177n Hardware Version:Archer VR900 v2 00000000

The firmware it came with did not work with IPv6 but after upgrading it kind-of works.

Because I am a BT BB customer and they require an IPv6 prefix of /56 the VR 900 only has a /64 preset available and it can't be altered. As I understand it some other UK ISP's are similarly affected. It seems TP-Link are in the process of building a firmware update for the V2 but no mention of the earlier V1.

Most ISPs require a /56 prefix externally but you should also have been assigned a different "subnet" with /64 prefix for your PTP network. For my Archer VR900 v2 I simply enabled IPv6 and SLAAC on the WAN interface and internally enabled RADVD. The router picked up its /56 prefix address from my ISP and started assigning the /64 prefix subnet to internal devices. I could also set the internal site prefix type to static (from delegated) and set the PTP subnet with /64 prefix for the LAN, but since my ISP was providing that anyway to the router it mode no difference.

That said, IPv6 support for the Archer VR900 v2 is clearly broken. Apart from having to upgrade the firmware to get any IPv6 working, it only works for short periods. It starts and stops with no intervention. One minute working fine, next minute the router stops routing IPv6 packets. The external interface still responds to ping6's, but the internal LAN stops responding to the Global IP. Only pings to the local link IPv6 address (i.e. what it tells hosts the default router address is) work.

Thanks for the input ams I may have a go at following your details but it seems that unless new firmware is released it may not be the long term solution.