ZTE does not publish firmware for this device on their public website. Unlike consumer routers (like TP-Link or Asus) where you can download firmware freely, ZTE ONTs are OEM products . They are custom-built for ISPs (like China Telecom, PLDT, Claro, Etisalat, etc.).
If this sounds too complex, the device is e-waste. Buy a replacement. zte zxhn f670l epon firmware link
for technical documentation, though direct binary downloads often require a partner login. Version Identification : Visit the ZTE Terminal Support Page ZTE does not publish firmware for this device