Your router should be plug and play, even if you do not have the cd. But you mention you are running the Linksys through another router as well. Problem there is, both routers are fighting to distribute the IP address’s to your network and confusing things. You need to log into your linksys and change it from a gateway to a router, so it gets its IP address from the first router. Can you just swap out your old router for the new linksys? Or do you have a bunch of computers already wired on the old router? Depending on your ISP, you will need to power down your cable modem or DSL box before you plug in the Linksys. You might have to also call your ISP to tell them about the change if you have a static IP.
To get to the administration page on your linksys, go to your browser and type 192.168.1.1 do not put anything in the name field and just type admin in the password field. This should get you in to make changes without the CD.
You can jump over to https://www.dslreports.com/forum/linksys and see if anyone over there can help.
Hope this gets you started in the right direction…..
Andy