We are sorry to know that the device iGS618 can't be connected with APP.
To fix the problem:
Firstly, pls help to check if the Bluetooth function is on or not at teps: Sensors>Blutooth.
Secondly,pls try to remove the device in APP then reconnect again. Navitating,click the device>remove the device.
And you may also see articles to troubleshoot the problem: