Unfortunately, Nck Client is not supporting the Nokia Lumia 521 device. You can find all the supported devices on the official web page of the developer. In order to unlock the Nokia Lumia 521 mobile phone, I recommend you check the instructions and guides that you can find using Google search engine.
How can I unlock my Nokia Lumia 521 device?
Unfortunately, Nck Client is not supporting the Nokia Lumia 521 device. You can find all the supported devices on the official web page of the developer. In order to unlock the Nokia Lumia 521 mobile phone, I recommend you check the instructions and guides that you can find using Google search engine.