If the key you are supposed to use from your ISP doesn't work, I would call your ISP. If you feel you need a username/password to login to your connection, and you don't know the credentials, call your ISP.
Get the theme here
Things not working is one thing, but not having the proper information to get things to work is quite another.
I don't know why you would change anything in regards to IP on the router itself. Most routers will essentially have a static IP, but more along the likes of 192.168.x.x - whereas it sounds as if you are trying to assign your public IP handed out by your ISP, to your Router.
What I would recommend is you disconnect your linksys for now, hook your PC into your Westell directly, and then call your ISP. At that point, if the connection isn't working it's their problem to figure out. Not trying to be direct you elsewhere, but they would know the proper configuration and settings to get you up and running.
At that point, they may/may not help you get the router to work. But we could always help you with that