A mobile plan is not required, but you do need LoRa reception. You can ensure this yourself (with 1 or more LoRa gateways), use one of the public networks such as TTN or Helium (you need to check the coverage in the desired location) or use the LoRa network of Swisscom, which has good coverage throughout Switzerland. With the Swisscom and Helium network you need a plan