Hi!
This has been discussed here, 3.19 and Routed networking mode configuration example needed
Just after that reply by @tomp was a post (now withdrawn) by @MSTY that still, these instructions still do not work on Ubuntu container images. Then, @tomp replies, but the withdrawn post makes the conversation difficult to follow. In effect, the issue is that netplan on Ubuntu container images really tries to do the default, which is DHCP on the interface. You need to replace (in the container) the file /etc/netplan/* to the version found in that thread. You can do that either manually (lxc file push), or you can pass the information to the container through cloud-init.