Many ISPs flash their own firmware with a different password (e.g., Vodafone, O2, Telcel). Check the sticker on the modem itself or its packaging – sometimes the password is printed as admin + IMEI suffix or a random string.
Unlike older router models that utilized a universal default password (e.g., admin / admin ), the ZTE MF79U employs a security measure where the default Wi-Fi and Admin passwords are randomized per device. zte mf79u password