Question about Garmin HRM-Pro Plus

Hi everyone, I wanted to use my heart rate monitor and see it appear in the Garmin Connect app. However, every time I try to pair the HRM-Pro Plus strap with the app, the pairing fails. I've tried restarting my iPhone, restarting the Garmin Connect app, removing the battery from the HRM-Pro Plus strap (wait 10 mins) and then reinserting it, etc. Somehow it doesn't work with the pairing. Has anyone else had this problem? And can you help me? Thank you very much!