Only way it to do a factory reset at the back of the router there's a Reset button.
Unless you set up password recovery and can remember the details you entered. But even that has problems.
If password recovery is enabled, but you have not set the answers to the questions, it keeps sending you back to the beginning of the process. If you haven't already given the thing your personal answers, they are not in its memory and it has no way of knowing if they are correct.
As @microchip8 says, if the standard factory password (password) does not work, then you will have to factory reset the thing to get back to that. Then either disable password recovery (check the manual for your device) or provide some memorable answers to those questions.
