Look under the battery or on the USB dongle's casing. It will be a 15-digit number. Write it down.
The "Online Calculator" is not a tool that hacks the modem in real-time; it is a client-side or server-side implementation of a reversed-engineered algorithm. Zte Modem Dongle Unlock Code Calculator -16 Digit- Online
This tool works with a wide range of ZTE USB modems, including but not limited to: Look under the battery or on the USB dongle's casing