...But you don't say what the problem actually is?
Assuming it's not getting an IPv6 address at all, it would suggest you need to set the TP-Link on "Native IPv6" or "PPP Ipv6".
BT use /56 Prefix Delegation
It won't work fails to connect IPv6 settings are the following
IPv6 enabled
PPoE
Share ipv4 session
Get IPv6 is auto
Prefix delegation: enabled
DNS : set to cloudfare
I'm not particularly familiar with this TP-Link model myself but I seem to recall another post were it was the share IPv4 session that was causing the problem.