Hi chaines,
Please check: this topic
Use /32.
Host:
lxc network set lxdbr0 ipv4.routes PUBLIC-IP/32
Container:
ip -4 addr add dev eth0 PUBLIC-IP/32 preferred_lft 0
The traffic goes through the host, so a virtual mac is not necessary. I also had problems with this, but this option still works in 18.04 with LXD.