External HRM keeps disconnecting

I bought an Instinct watch and HRM-Dual strap for running. The watch is able to find the HRM and connect it via both ANT+ and Bluetooth. However after it has connected, I'm able to see my heart rate on the watch only for few seconds. After that the heart rate drops and HRM is disconnected from my watch.

If I navigate back to "settings -> sensors & accessories -> hrm-dual" it says disconnected, but after navigating there, it quickly changes to 'searching' and to 'connected'. After that I'm once again able to monitor my heart rate only for few seconds until it disconnected again.

Link to a youtube-video that describes the problem: www.youtube.com/watch