There are several possibilities to enter your home without a smartphone:
You can still use your existing, mechanical key.
You can ask another authorized person (friends, family) to open the door for you. If you have a Nuki Bridge, this even works from afar.
You can use the Nuki Fob. The Nuki Fob is a small Bluetooth device with which you can lock your door without a smartphone.
You can use the Nuki Keypad. Open your door by entering a six-digit entry code.
At a power level of 20% Nuki will notify you via the Nuki app and directly on the Smart Door. So there is enough time to replace your batteries of your Smart Door. More information regarding the batteries you can find here.