If your router cannot connect to the update server or you have a specific firmware file, use this method. How to update the system? - Support - ZTE

If you want, tell me your ZTE router model and whether it was provided by an ISP and I can outline exact download locations and the likely menu path for the web UI.

Write down or take a screenshot of these alphanumeric strings. You will need to match these details precisely to avoid installing the wrong software, which can permanently break ("brick") your device. Step 3: Backup Your Existing Settings