Please post the results of "route -n" in [code][/code] tags. My routing table looks like this:
Your results should list "eth0" rather than "wlan0". My computer's IP is 192.168.100.110, and the gateway is 192.168.100.1. I see other computers in the 192.168.100.0/24 network via broadcasts; all other traffic goes to the gateway.
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.100.1 0.0.0.0 UG 0 0 0 wlan0
192.168.100.0 0.0.0.0 255.255.255.0 U 9 0 0 wlan0
My network's topology is rather similar to yours because my gateway router is behind my Internet-facing router. I can see the Internet from all my machines because both routers do "masquerading" and impersonate the machines behind them. The router at 192.168.100.1 masquerades the traffic from 192.168.100.0/24 as if it is coming from its upstream-facing IP of 192.168.1.2. The Internet facing router has local interface 192.168.1.1 that talks to 192.168.1.2 and masquerades its traffic again using the public Internet address on its "WAN" side.
Try this experiment. Connect with the Mac and identify its IP address and the gateway's address; let's say these are 192.168.1.10 and 192.168.1.1 respectively. Now disconnect the Mac from the network. On the lubuntu box, use these commands
How are we doing? Can you connect to the router's configuration page now?
sudo ifconfig eth0 down
sudo ifconfig eth0 192.168.1.10 netmask 255.255.255.0
sudo ip route add default via 192.168.1.1
Oh, just one other thing, did you reset the router to its factory defaults after borrowing it? If not, I would do that first. There usually is a small switch behind a hole in the back of the router that you can depress with a paper clip. Turn off the router, then turn it back while holding down the switch. Hold it down for 30 seconds or so, then turn off the router and turn it back on again.