Netplan is (IMO) another “systemd” approach (read as “fuckup”) to next linux component. Maybe it is ok for desktops, but after tinkering with it on 17.10 server i throw it away. Multiple network devices (mix of 10ge, 1ge) bonds, tunnels, vlans - it is pain in the ass and when i netplan apply - everything stopped for a while - i dont know how to manipulate only with one interface without affecting others.
If you want “old” eni behaviour back you must install server with
kernel option netcfg/do_not_use_netplan=true
and preseed
d-i netcfg/do_not_use_netplan true
With this - installer install ifupdown same way as previous versions of ubuntu server and netplan isn’t in action at all.
Or try to install ifupdown to your existing “netplaned” server…