This is an old topic. But it seems to get hit on the google search pretty often. What people are probably failing to do is properly reset the device.
On the back of the Cisco DPC3825 there are actually TWO buttons. There is the wireless setup (WPS) button that comes on most all Cisco/Linksys devices nowadays and a reset button. If you look right next to the WPS button, there is a RECESSED reset button. You will need a pen or other similar object to press the button.
With the router powered on, press the reset button with your pen and wait for the lights to change to orange. Once they're orange, remove your pen and wait for the gateway to boot up all the way. At this point you can reach the Gateway management page at http://192.168.0.1/
The username and password will be blank, just press login. Once you're inside it's highly recommended you change it, AND WRITE IT DOWN SAFE.
These tips are based on the Cisco DPC3825 Gateway from Mediacom, some sources on the internet report that some of the devices from cable providers outside of the US, have different default passwords.