Issue with 2 wireless networks and one cable modem?

I have a cable modem and two wireless routers. One of the routers is servicing one area of my house. A long ethernet cable is connecting the first (LinkSys) wireless router to a second (TrendNet) wireless router in another area of the house. Both wireless networks function well, but I have two problems:

- I would like to have my iMac hard-wired to the TrendNet wireless router. When I connect an ethernet cable to it, the iMac gets a self-assigned IP and cannot connect to the internet. I need the iMac to share the same subnet as the TrendNet router to use it with my AppleTV.
- When I try to configure the TrendNet wireless router, the browser loads the LinkSys config page.

Any suggestions on how I can get this network to function better? Should I have third router splitting the Internet feed to the two wireless routers?

Thanks.