There are 2 methods to restore default settings.
Method 1: To reset the device, begin with the unit powered up and with a solid Green Power light, then using a small tool depress and hold the Reset button on the back of the device just until you see the color of the Power light change from Green to Amber, and then release it before it turns Red. The length of time that the Reset button must be held to accomplish the reset varies from model to model, but as long as the Reset button is released while the power light is Amber the Reset will be successful.
Method 2: Log into the device, click on Advanced, click on Configuration, click on the Restore button next to "Restore Modem to Factory Default State". The modem will reset and reboot to factory defaults.