You cannot use a floating IP managed inside the container with routed NIC type.
As the name suggests, it uses routing to achieve network connectivity with the external network, rather than sharing the layer 2 domain (such as with macvlan or bridge NIC types).
Infact one of the important features of the routed NIC is that it prevents containers from announcing (and thus attracting traffic for) IP addresses that have not been explicitly allowed by LXD.
You probably want to be using a bridged NIC connected to an unmanaged (manually setup) bridge that is also connected to your external network.
This will then allow your container to send APR/NDP packets to advertise it is responsible for additional IPs.