I believe it is something to do with this /56 prefix because even with prefix delegation enabled it keeps flipping from an fe80 to a 20 address and it says /64 on the end, it's totally glitched.
(they just need to allow users to define the prefix instead of detecting it which obviously isn't working correctly).
Nicely spotted, I don't think it'll make a difference, corrected nonetheless.
So I'm actually making some progress, using "non-address" on WAN side and DHCPv6 on LAN side, and a couple of reboots.
One problem however, it's assigning 1 IPv6 Address to 2 devices at a time, (I suspect because of the /56 prefix BT uses and the AX95 still detecting /64).
So only half of my devices are getting IPv6 functionality and able to test on test-ipv6.com 🙄.
TP-Link support is going to try sort it out (we'll see, IMO it needs a firmware update).
Also, if anybody else has this issue, this is what seems to be working for me at the moment:
it did require a reboot upon updating the settings, otherwise it doesn't work.
I need to keep an eye because it assigned the same v6 address to two devices simutaneously which meant only one of them worked on test-ipv6.com (might be to do with the prefix), will keep an eye and report back.