The ZTE ZXV10 W300 is very old (WiFi 4, ADSL2+). Instead of risking a patched updater:

The official ZTE updater (ZXV10 W300 Firmware Updater.exe) refuses to flash anything that isn't signed by the original ISP. If you try to load a universal firmware, you get errors like:

The patched version removes these checks. It lets you:

Do not blindly download from forum posts or file-sharing sites. If you still want to search: