Good question. I just did that and asked their support what can I do to enable it. They replied, like always with me, in less than 20 minutes but .. with an outdated tutorial. On Ubuntu 18.04 ifupdown package was replaced with netplan.
So .. I adapted it like this:
- ran “crontab -e” to edit the crons
- put, at the top the following line:
@reboot ip addr add YOUR_NEWLY_ORDERED_IP/32 dev NETWORK_INTERFACE
What does that do? On every reboot it adds your additional IP to be served by the network interface (in my case enp4s0).
Of course, if you don’t want to restart right after editing the crontab .. well, just run the command without the “@reboot” part.