The T-Mobile Device Unlock app is an Android app allowing you to request a SIM unlock directly from your device. There is NOT a Device Unlock app for iPhones!
Also, you cannot just install the T Mobile Device Unlock app on your device. If your device supports it, it will be PRE-installed. You cannot go to Play Store and just install it
If you cannot find the Device Unlock app on your device, do not get frustrated immediately. Make sure that the app is not disabled or hidden.