Adding additional IP addresses in OVH is rather unconventional and the guide they point to in their mail is non-existent. The process, fortunately, is simple:
- ssh to the server as root
- Add the new IP address after the original IPv4 Address
DHCP=no Address=Main_IP/24 Address=Failover_IP/32
- Save and close the file
- Reboot the server
You should now be able to ssh to the server using the new IP Address (which OVH likes to call Failover IP Address) in addition to the old one.
Repeat the process for any additional IP addresses.
Note: I have tested it on two different OVH Servers (in France & Canada) for Ubuntu 16.04 and Ubuntu 18.04.