Additional IP not on the same IP block as gateway

Additional IPs I got from OVH is different from IP block that my main IP is on. And this is what they want me to configure the additional IP per their help ticket.

/etc/network/interfaces
auto lo eth0
iface lo inet loopback
iface eth0 inet static
address IP.Fail.over
netmask 255.255.255.255
broadcast IP.Fail.over
post-up route add Your.Server.IP.254 dev eth0
post-up route add default gw Your.Server.IP.254
post-down route del Your.Server.IP.254 dev eth0
post-down route del default gw Your.Server.IP.254

I was wondering how could I configure that when I am running a KVM and a ISO net install for Ubuntu. They won't allow me to go forward the installation as they said gateway is not reachable and I have no way to set the "post-up" route add.

Thanks