ZTE MF91 Drivers

Launch DC-Unlocker client software

Open Server tab, enter your username and password that you received after credits purchase. Click "Check login". (you can buy credits here www.dc-unlocker.com/buy).
If DC-Unlocker/Vygis/Rocker/Infinity dongle is connected, do not enter username and password, only click "Check login"

Select manufacturer: ZTE modems, model leave Auto detect (recommended).
Press "Detect" button (Big magnifying glass)

Wait till DC-Unlocker fully detects device

If you get "modem not found" error message, run DC-Unlocker as Administrator and perform detection again.   Make sure drivers of the device are installed as well - ZTE MF91 Drivers

Right click on DC-Unlocker icon and select "Run as.." or "Run as administrator" from options menu to launch software with administrator rights

Open Unlocking tab, click “Unlock”

Wait till you see a message that modem is unlocked

After you see that modem successfully unlocked, reset it. Now you can use it with any SIM card.

Note: if battery indicator is missing after unlocking device check procedure to repair it here