When you did the initial setup, you should have created two passwords: one for connecting to the WiFi, one for logging into the management console. Please make sure that you are using the password for logging in the console.
If you are using the right password but still cannot login, you can reset the router:
1. With the router fully powered on, use a pin or pen tip to press down the reset button (next to the power connector). Hold it for 10 seconds then let go.
2. Wait for a few seconds, all LED lights will flash once and the router will reboot.
3. After the router is fully powered on, connect to it and do the initial setup again.
Do not disconnect power during the above process.