The best variant is to additionally mount a keypad outside with a secret code chosen by you - then you can always open your door - should the mobile phone be empty, lost or, we hope, not stolen. The same function can also be performed by the Nuki FOB, but then you would have to take it with you or leave it somewhere (e.g. in the car).
Of course it also works "old-school" and you leave an emergency key somewhere in the bushes, but this only works if your cylinder can be unlocked from the outside when the key is inserted - this is called the Prio function here in Switzerland.