The ZTE MF90 is a classic, robust 4G LTE mobile Wi-Fi hotspot. However, many of these devices were sold through specific network carriers. Carrier-customized firmware often locks the device to a single network, hides advanced settings, and delays critical updates.
If your MF90 is just carrier-locked or has custom boot logo:
Easy, manual setting of Access Point Names (APN) for various carriers. Preparation Before Flashing