The route add command will only put a route in the routing table for as long as the machine is not rebooted - you need to edit the network configuration files on your machine to add the static route.
These files depend on what flavour of UNIX you're using. In HP-UX it's the netconf file under /etc/rc.config.d - every version of UNIX will have a network configuration file which will be easy enough to find.
You just need to add the interface name, IP address, Gateway and subnet mask as a separate static route instance.