Are you trying to or just update security ? Do you have your ISP's PPPoE credentials and VLAN ID handy?
Advanced users looking to "repack" or modify their F671Y often use specialized tools to decrypt the device's configuration: ZTE Config Utility : Many developers use the zte-config-utility on GitHub to attempt to decrypt config.bin zte f671y firmware update repack
: Use appropriate tools (e.g., 7-Zip, firmware unpacking tools specific to ZTE) to unpack the firmware. This usually involves extracting the contents of the firmware file to a folder on your computer. Are you trying to or just update security
Note: Some ISP-locked models also have a digital signature (RSA). You cannot bypass that without signing keys – repacking will fail. zte f671y firmware update repack