DD-WRT may request a username and password, which by default is root/admin (which you should change to your own if this happens). Newer versions will ask you to choose your own.

Installing DD-WRT can be complicated, but the most common method includes using the “upgrade firmware” functionality built into your router’s default firmware.

Open source DD-WRT or OpenWrt firmware can breathe new life -- and advanced features -- into your old wired or wireless router.

Basically, what I want to do is use my DD-WRT'd WNR3500L router to provide a connection *only* to the internet, isolated from the rest of my network, to one port, and have the other ports be ...

Set up a full IPSec tunnel to someplace. I don't know if dd-wrt has this functionality or not, but that would be a way to do it. Have DNS requests, etc, go through the tunnel.