Reset to default, by holding down the reset button inside the small hole on the back next to the USB port on the back as shown in your
MANUAL -- use a paper clip and hold it for 10 or 15 seconds. Then login using the default settings, and then re-enter your settings.
You can only login to your LAN default gateway (on your router), not your WAN default gateway address (that is your ISP), and the LAN default gateway should be something like 192.168.x.y where x and y are from 0 to 254.