In that case if can dedicate the entire subnet (or a /24 within it) for use by LXD you can create a new managed LXD bridge and assign it that subnet and disable NAT, e.g.
lxc network create mybridge --type=bridge ipv4.address=104.218.16.146/21 ipv4.nat=false
Then you can connect instances to the network:
lxc config device add c1 eth0 nic network=mybridge
Alternatively you can route single IPs from that subnet into your instances using a routed NIC see How to get LXD containers get IP from the LAN with routed network