ZTE has since removed the 2021 files from their public FTP to push newer (and arguably less stable) 2023 builds. However, the Internet Archive and several OpenWrt forums have preserved them.
A recurring theme in discussions about the ZTE H2640's firmware is that it is . The Italian tech forum hwupgrade.it , which hosts an official user thread for the device, explicitly states, "At the moment the firmware file for the local update is not available. The firmware update is done exclusively by the provider". zte h2640 firmware 2021