IPV6 on BT Infinity

Has anybody managed to get IPV6 working with an Orbi and BT Infinity.  I have the Orbi in Router mode running off a TP Link VR900 in bridge mode as my modem.  I understand that BT use DHCPV6 PD/56 but see no way of selecting that on my Orbi.  If I let it Auto config or detect (What is the difference?) nothing happens. And what is teh real world difference of leaving IPV6 on the disabled setting?

Thanks for any advice.