Initially good function but fail after a short time

I bought this product for my iPhone, the build quality of the heart rate monitor and easily separable via buttons belt is very good.
The connection to the device worked very well at first, but worked the connection after some time no longer reliable. A battery change brought no remedy. I have the device with the app Runtastic used for a coupon code included. Apple Health also no longer reliably coupled to the belt after 3 weeks use.

I then get a replacement device, but was unfortunately observed the same course, so I have seen the product then finally returned.

I can not say for sure whether it is the product itself or the iOS Bluetooth connection, however. Has been tested with an iPhone. 5