The Bluetooth connection is used for collar set-up, fence creation, and while you train your dog. Keep the collar and phone within 50 feet of each other during these activities. Similar to a Bluetooth speaker, if the collar and phone are more than 50 feet apart then Bluetooth will disconnect.
Move the collar and phone to within a few feet of each other and be sure Bluetooth is activated on your phone. Open the app, go to the Dogs screen, then tap Connect to Manage.