When LunaR watch loses connection over a certain time (which you can set in the App-Settings-Scan duration), watch Bluetooth will be automatically off. Once you came back to your phone and want to reconnect, make sure the LunaR App is on, you can press any button on the watch to trigger the BT on, and it will reconnect. 

If not, please check if LunaR has been secretly connected to other phones on phone BT(this happened most of the time); or if LunaR App has been "killed" by phone system.