|Which password won't reset?|
If you mean the one that allows you to login to the management interface then either you're not logging in correctly or, you didn't actually reset the unit properly.
Get your manual/setup guide. It should detail both procedures. Pretty much all of the router's I've worked with lately are reset by powering the unit up and once it's done booting you press and hold the "reset" button for 10 seconds. Release the button and the unit will reboot. Once done rebooting it will be reset to factory defaults and you can then reconfigure it.
To the best of my recollection every SOHO Router I've ever worked on had one of two ways to login to it when it's at defaults.
Password: blank (as in type nothing in this field, just put admin in username and hit enter)
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.