Hi Anshumaan
So what you actually mean is, when your phone is out of Bluetooth range, the App loses connection to the device and when back in range, it does not reconnect.
What BT are you using, Classic or BLE? If BLE, the next edition of the MIT BLE extension has a solution for your issue. If using Classic, you will have to alert the User about the loss of connection (suggest a sound) and unfortunately they will have to repeat the connection process.
If this is going to happen too often, then either a hardware solution (an intermediary device for example) or a change of comms (WiFi?) could be considered…