Connect device to PC via USB and install everything that comes in it's drive.
Turn on the device.
Make sure WEBUI of device on browser is closed.
Launch ZTE Airtel WD670 unlock tool.
Enter your username and password that you've received after credits purchase. Click "OK". (you can buy credits here www.dc-unlocker.com/buy).
If DC-Unlocker/Vygis/Rocker/Infinity dongle is connected, log in step will complete automatically.
On software window check WD670 WEBUI Login details. By default user and password for it is admin/admin. If device's log in details were changed, you should enter them before proceeding further.
Press "Unlock" and monitor the process
Software will indicate when unlock completes.