You need to have a NFC phone to use your phone for 'local' NFC unlocking. Please make sure NFC is switched on with your phone and move the back of the phone around the black cover of the lock which has the NB IoT logo until you hear a beep sound (and you find the NFC sweet spot on the back of your phone (its usually in the top third of the phone). 

Also the digital keys app must be open to use the NFC phone unlock feature (or running in the background). You don't have to hit any unlock buttons on the phone, just make sure the app is open or running in the background to use the NFC unlock feature.