You would need to set up a static route so any requests to the servers IP are routed towards that networks router. So for example if your server is 192.168.1.250 and the router on that network 192.168.1.1 with a subnet of 192.168.1.0/24 - you would require something along the lines of.
route -p add 192.168.1.250 mask 255.255.255.0 192.168.1.1
You would also require a second static route for everything else to be defaulted to your internet router. So for example if it was 10.0.0.1
route -p add 0.0.0.0 mask 0.0.0.0 10.0.0.1
I haven't used static routes in a while so I'm a bit rusty and this might be wrong. It's worth doing some revision on Static Routes yourself.
Good place to start -...