ZTE offers a Device Manager tool to help users manage and recover their devices. To use this method:
If you've never changed the password before, the default login credentials are: zte f6640 password
If you are certain the password is correct but still cannot log in: ZTE offers a Device Manager tool to help
Based on standard ZTE firmware and common ISP customizations for this model, the following credentials are most frequently used: zte f6640 password
Use a paperclip to press and hold the button for while the router is powered on.
If you've forgotten the admin password, you can reset the router to its factory settings: