I can not understand why on device from year 2019 it is no possible to synchronize it with phone during Activity.
Lets imagine I am driving 100 km ride and somewhere in the middle of ride I need to use navigation. Great, I have Garmin Connect Mobile. So I am creating course on my phone. After that Garmin Connect is trying to send this new course to my device. But it is not possible until I stop my activity 
So I need to stop activity. After stopping activity it is automatically send (in my case) to Strava. If during long ride in unknown region I need to create course on my phone many times my daily activity is divided on many small rides.
BR
Arek