Look at the bottom or the back of the ZTE F6640. You will find a sticker containing: Usually 192.168.1.1 or 192.168.0.1 . Default Username: Often "admin".
If these don't work, you might need to check the underside of your device or the user manual for the default login credentials. Sometimes, the information is also printed on a sticker. zte f6640 admin password