Your sensor may be paired with the iOS device itself but not with OTO application. To resolve this issue, proceed with the following:
Power on the sensor by attaching it to your chest strap on your chest (the blue light on the sensor will start blinking)
Open the Bluetooth Settings in your iOS mobile device and ensure that Bluetooth is turned on (if not then this could be the cause of the problem)
If Bluetooth is turned on already, then continue with the steps as follows:
In your iOS device’s Bluetooth settings you should be able to find the activated OTO sensor, it’s called “OW-CB2”
Click now the little “i” icon next to “connected”
Click now “Forget this Device”
Please proceed now to the OTO application
Tap on the sensor image on the top left corner of the application and click on "Search for Device" to proceed with the sensor pairing within the application
You may repeat this procedure to ensure that the sensor is not paired with your iOS device but with the OTO application.