In order to reset your Pokéwalker without a DS, you will need to connect it to a computer and use the official Pokémon Home software. This can be done by opening the Pokémon Home app on your computer, and then selecting the WirelessConnect option from the main menu. Once you are connected, select the Pokéwalker option from the devices menu. From here, you will be able to reset your Pokéwalker by pressing the Reset button.