If your ISP is unhelpful, a few technical forums maintain vetted firmware collections. These are not "official" but are used by thousands of technicians:
Manual firmware flashing is often restricted by ISPs. Some users have reported being locked out of manual updates after receiving provider-specific OTA (Over-the-Air) updates. For official support, it is recommended to visit the ZTE Support Portal or contact your ISP. zte zxhn f670l epon firmware link