How do I can connect HRM-DUAL to Garmin Connect
You connect the strap to a smart watch or bike computer , phone app (strava , wahoo fitness, etc) , treadmill or bike console , zwift / RGC virtual bike on computer/phone.
Garmin connect app is for viewing activities and health info recorded with Garmin watch/bikecomputer or imported from 3rd party apps (iFit , MyFitnessPal, Zwift , etc) or files that have been manually uploaded (.fit , .gpx, .csv, etc)