Re: 2 routers and one modem?
If it's a router with WAN on one side and LAN+WLAN on the other side, try to connect a LAN ethernet with the LAN ethernet side of the other. Be sure the IP addresses of the routers are different but within the same subnet and DHCP server is enabled on only one of the routers. If it's an old router, you might need a crosscable.
Otherwise connect a LAN port on the N router to the WAN of the G router, but then you have an additional network with Network Address Translation, unless you can reconfigure this router into Bridge mode.
In the NAT case, the N-router connected systems cannot connect to the G-router connected systems without