If the password is lost, the only recovery is a factory reset:
Physical button (recommended): with the router powered on, insert a pin into the RESET pinhole on the rear panel and hold for 5–10 seconds until the LED flashes.
Via the Web UI (if accessible): System Tools > Backup & Restore > Perform Restore Default.
Warning: A factory reset permanently erases ALL settings. Back up first via System Tools > Backup & Restore > Backup configuration files if the Web UI is still accessible.
Tips: To change the password without a reset: System Tools > User Management. Enter the old and new password (max. 16 characters) and click Save & Apply.