A care coordinator working from their home or office that has access to the internet via Wi-Fi or Ethernet cable would not need a cellular data plan. However, for a mobile technician or a veterinarian, it is highly advisable. Without access to the internet you will be able to use REX, though the REX offline feature is designed to be used for short periods of time. In the offline mode, REX is not able to sync between all the devices used by other users in your practice and therefore when offline you will not have updated information about your latest appointment or recent changes to a patient record made by another user. Also, without syncing REX would not be backing up your data.