Either the routers aren't interfacing with the wireless ethernet thingies (broken ethernet port, wrong ethernet port, the wireless ethernet whatevers are faulty in some way) or there's something blocking or otherwise interfering with their transmission
No, the ethernet recievers work just fine; I can access both routers config pages using either network, it's just that internet access doesn't work on the Netgear router. I suspect that the D-Link routers firewall is somehow blocking the Netgear router, but I can't find anything in the settings or logs that suggests it.
Well, I can't really tell too much.
Make sure your primary router is set the the 802.11n standard, as it's backwards compatible with b and g, the only settings your secondary router will run.
If everything else is properly configured then my only guesses are the wireless ethernet link isn't working right, or your links aren't connected to the proper port. Not sure how these ethernet things work, but try connecting one to a laptop in the area you're trying to place the second router and test the connectivity there, so we could rule out the possibility of a faulty link.
The D-Link router is set to use both n and g (which aren't interchangeable), and the Netgear router uses both g and b (atm, will disable b soon). They both use different SSIDs, and are therefore seen as different networks even though they are on the same subnet (hooked up LAN port to LAN port, as seen on the chart I made), and are also on different channels.
While we're on the subject, is there a way to make the routers act as a single router, ie use the same SSID? I read in the comments on the guide I used that one guy was able to do this by simply naming the SSID's the same, but when I tried this the wireless radio on the Netgear router just turned itself off. Is there any easy way to achieve this, or am I out of luck?