This will erase ALL settings (IP addresses, user accounts, recording schedules, and possibly video data depending on the model).
If you didn't set a recovery email or are offline, you can perform a hard reset using the physical button inside the NVR. How To Password Reset Dahua NVR dahua nvr password reset
This is the most common method for modern firmware. It requires a recovery email to have been linked to the account during setup. This will erase ALL settings (IP addresses, user
Open the (formerly gDMSS/iDMSS) on your smartphone. dahua nvr password reset