You will have to reset the router to factory settings. There is a small hole in the back, depress the microswitch with a paper clip when the power is on. The default password is 1234. Some of these devices are general, while others are hardwired to work with certain providers. So if, say, this 660 is branded with embarq it is unlikely that you can set the parameters to work with other providers.

I should mention that if you care about security you should not use WEP - a cracker can break an internet connected WEP device in minutes with the right toolset. The protocol has many problems that simply can't be fixed, period. Consider using WPA2 with WPA backcompatability, as allowed by the 660.