While the Garmin Connect app is in the foreground (shown on the screen) it works but not when it is in the background.
I got it working by enabling
"Display pop-up windows while running in the background…
With thanks to @lgroth! I have a Pixel XL with settings navigation slightly different:
Settings | Apps & Notifcations | See all xxx apps | Connect | Advanced | Display over other apps | Display over…
I do not have your device to to make tests.
My view is that while installing the Garmin application is not requesting the needed device permissions for it to work properly. There could also be other incompatibilities. The Garmin application needs to be updated. Garmin needs to test the application on as many devices and Android versions as possible.
A suggestion for people who have enabled all permissions and are still having problems: if you go to Settings -> Apps -> Connect -> Permissions, you will NOT see ALL permissions for the app, only basic permissions. Make sure "Allow background activity" is set on in Settings -> Apps -> Connect -> Battery (mine can't be disabled, but that may be a function of my specific Android version and distribution). Also, go to Settings -> Apps and hit the three-dot menu and select "Special access" or "Special permissions" - this has all of the advanced permissions settings that are not necessarily displayed in each app's specific entry. Try enabling anything Connect doesn't have enabled there to see if that resolves the problem.