A common issue with Debian-based distros (Ubuntu included) is that the default network configuration won't allow a gateway outside the subnet, which is exactly what OVH needs. Here's how to fix that.
Consider the following:
- Your main IP is A.B.C.D
- Your FO IP is E.F.G.H
On your Ubuntu VPS, Put the following on /etc/network/interfaces:
iface eth0 inet static
address E.F.G.H
netmask 255.255.255.255
broadcast E.F.G.H
post-up /sbin/ip -4 r a A.B.C.254 dev eth0
post-up /sbin/ip -4 r a 0.0.0.0/0 via A.B.C.254 dev eth0
pre-down /sbin/ip -4 r d 0.0.0.0/0 via A.B.C.254 dev eth0
pre-down /sbin/ip -4 r d A.B.C.254 dev eth0
Of course, replace the letters with corresponding IP. Also, if your virtual NIC is not eth0, replace it to match your virtual hardware. Then, restart networking("/etc/init.d/networking restart") or reboot the VPS. That should do the job.