Comment 11 for bug 1771886

Ugly workaround until this bug is fixed, but this seems to work with Ubuntu 20.04.3 LTS and 21.04. It's executed at startup, e.g. from rc.local, as root; br0 in this case is the interface to a WAN network so adapt to your system before running.

echo "IPv6SendRA=yes" >> /run/systemd/network/10-netplan-br0.network
echo "DHCPv6PrefixDelegation=yes" >> /run/systemd/network/10-netplan-br0.network
echo "IPv6PrivacyExtensions=yes" >> /run/systemd/network/10-netplan-br0.network
systemctl restart systemd-networkd
/usr/lib/systemd/systemd-networkd-wait-online

Yes, it's ugly and am sure breaks a whole lot of Netplan's goals, but at this point is Netplan that breaks the goal of having a prefix-delegated network so it's a (much) lesser evil IMHO.