Setting up host addresses for IPv6 is tricky.
host ubuntu Server_Name {
host-identifier option dhcp6.client-id Long_ID_String;
fixed-address6 IPv6 address;
}
How to get
host-identifier option dhcp6.client-id?
I opened the DHCPv6 server.
sudo systemctl -l isc-dhcp-server6
Go the the server that you want to find the information on
sudo netplan try, enter, sudo netplan apply
Load sudo systemctl -l isc-dhcp-server6 again. You will see it on the screen. Copy the string into /etc/dhcp/dhcp6.conf for that server.
Sudo systemctl restart isc-dhcp-server6 or sudo reboot. do try and apply again and ifconfig to check