You should set ovn.ingress_mode to routed on your uplink network.
Additionally, LXD will advertise routes for networks that do not use NAT, if you’re using ipv4.nat, then the IPv4 subnet won’t be advertised, same for IPv6 with ipv6.nat.
External addresses/subnets directly added to an instance nic through ipv4.routes.external or ipv6.routes.external will also be automatically advertised through BGP.