You would have to open the port on both routers.
This is what i have on my modem downstairs, 1.10 is the address for the router upstairs, then for the router upstairs i opened 25565 for my computer which is connected with cable.Code:Local External IP Address Start Port End Port IP Address Start Port End Port Prot Description Enabled 192.168.1.10 25565 25565 0.0.0.0 25565 25565 BOTH test Yes
Hopefully you'll get the idea.