Can't get IPv6

[tam481]

Hello
I've got the following added to my /etc/docker/daemon.json file

{
  "ipv6": true,
  "fixed-cidr-v6": "fd21::/48"
}

I subsequently restarted docker (systemctl restart docker)

I then recreated my Matterhub container which is in host mode networking-wise but when I run docker inspect matterhub | grep -i IPv6

I get:

            "LinkLocalIPv6Address": "",
            "LinkLocalIPv6PrefixLen": 0,
            "SecondaryIPv6Addresses": null,
            "GlobalIPv6Address": "",
            "GlobalIPv6PrefixLen": 0,
            "IPv6Gateway": "",
                    "IPv6Gateway": "",
                    "GlobalIPv6Address": "",
                    "GlobalIPv6PrefixLen": 0,

The host has a ULA IP beginning with fd88::

What do I need to do to get IPv6 to work for matterhub?