If you can access the bootloader via serial or by holding the reset button while powering on (telnet on port 23 or 80), note the CFE output. The V2.5 usually has a protection mechanism that rejects any firmware without a valid signature header.
: The original firmware is built for "double play" or "multiplay" services, integrating a DSL modem, router, and wireless N networking into a single interface. WLAN Performance zte zxhn h108n v2.5 firmware original