TD-W9970 doesn't support 56 delegation.
You can contact support.uk@tp-link.com and ask tp-link support whether they have a beta firmware to support this.

I tried SLAAC instead of DHCPv6 but still same result. I have ipv6 addresses but no connection and status for the ipv6 is still connecting...

I managed to access the tenet port of the tp-link, Part of the output of :-
TP-LINK(conf)#wan show connection info

is
name=pppoe_0_38_0_d
{
enable=1
connectionStatus=Connected
possibleConnectionTypes=
connectionType=IP_Routed
PPPoESessionID=0
defaultGateway=217.32.141.3
name=pppoe_0_38_0_d
uptime=8326
lastConnectionError=ERROR_NONE
idleDisconnectTime=15
RSIPAvailable=0
NATEnabled=1
X_TP_FullconeNATEnabled=0
X_TP_FirewallEnabled=0
X_TP_IGMPProxyEnabled=1
username=bthomehub@btinternet.com
password=BT
PPPAuthenticationProtocol=AUTO_AUTH
X_TP_IfName=ppp1
X_TP_L2IfName=nas1_1
X_TP_BcastAddr=
X_TP_ConnectionId=0
X_TP_UseStaticIP=0
externalIPAddress=86.168.24.187
remoteIPAddress=217.32.141.3
maxMRUSize=1480
currentMRUSize=0
DNSEnabled=0
DNSOverrideAllowed=0
DNSServers=213.120.234.34,213.120.234.46
MACAddress=70:4F:57:64:E9:91
MACAddressOverride=0
X_TP_ClonedMACAddress=
transportType=PPPoE
PPPoEACName=
PPPoEServiceName=
connectionTrigger=AlwaysOn
routeProtocolRx=Off
PPPLCPEcho=30
PPPLCPEchoRetry=0
X_TP_SessionID=2510
X_TP_ServerMACAdress=00:30:88:00:00:04
X_TP_SecondConnection=None
X_TP_SecondConnectionIPAddress=0.0.0.0
X_TP_SecondConnectionSubnetMask=0.0.0.0
X_TP_SecondConnectionStatus=
X_TP_IPv4Enabled=1
X_TP_IPv6Enabled=1
X_TP_IPv6ConnStatus=Connecting
X_TP_IPv6CPUp=1
X_TP_IPv6AddressingType=SLAAC
X_TP_ExternalIPv6Address=::
X_TP_PrefixLength=0
X_TP_DefaultIPv6Gateway=fe80::0230:88ff:fe04:1388
X_TP_DefaultIPv6GatewayOverride=0
X_TP_IPv6DNSEnabled=1
X_TP_IPv6DNSOverrideAllowed=1
X_TP_IPv6DNSServers=2001:4860:4860::8888,2001:4860:4860::8844
X_TP_PDEnabled=1
X_TP_SitePrefix=2a00:23c3:3cbb:5600::
X_TP_SitePrefixLength=56
X_TP_SitePrefixPltime=-1
X_TP_SitePrefixVltime=-1
X_TP_DSLiteEnabled=0
portMappingNumberOfEntries=0
}

agree with uutommi.
My TD-W9980 is working with IPv6 after I get a beta firmware from TP-Link.