There are several possibilities to enter your home without a smartphone.
You can still use your existing, mechanical key.
Friends & family
You can ask another authorized person (friends, family) to open the door for you. If you have online access to your Nuki device, this even works from afar.
Good alternatives to open your door simply and quickly without a smartphone are the Nuki Keypad and the Nuki Fob.
Note: At a power level of 20% Nuki will notify you via the Nuki app and directly on the device trough LED signalization. So there is enough time to replace your batteries of your Nuki device.