I have Garmin Connect Version 4.36.0.25 on iPhone 11pro iOS 14.1. Many times the green dot who marks an active Bluetooth connection from the app to the watch goes away.
What I do is go to the settings of my iPhone and switch Bluetooth off and on. Then the connection will work for a certain time.
Sometime I get a red alert message: "Bluetooth permissen requiered. On you phone, please go to Settings > Connect and turn on Bluetooth."