The macvlan NIC type in Linux doesn’t allow LXD host <> instance communication.
So if you’re running the DHCP server on the host, the instances won’t be able to reach it.
If you’re sure you’re allowed to run a DHCP server that advertises onto OVH’s external network, then you could use an unmanaged bridge (Canonical Netplan) e.g. br0 and move your server’s existing IP onto the br0 interface, as well as setting up your DHCP server to listen on br0 address.
Then you could connect LXD instance NICs to it using lxc config device add <instance> eth0 nic nictype=bridged parent=br0