For OVH’s failover IPs, all you really need to do with a recent LXD (2.21 currently) would be:
lxc network set lxdbr0 ipv4.routes PUBLIC-IP/32
If multiple IPs, you can either use a CIDR subnet for the subset of additional failover IPs, or you can just set multiple individual IPs, comma separated.
Then in the container, all you’ll need to do is add a static IPv4 address, manually with:
ip -4 addr add dev eth0 PUBLIC-IP/32 preferred_lft 0
That line can be put as a post-up of your existing (DHCP) eth0 interface. The container will continue to pull a dynamic local IP from the LXD DHCP server but will then also have its public IP associated with it and will prefer it for outgoing traffic.