I bought a Garmin Forerunner 945 two weeks ago in Japan. I was unable to add the device to Garmin Connect Mobile, I tried all the recommendations read here and in the App's troubleshooting center.
I tried connecting the device directly to the phone and I was able to do that, I also connected headphones and Stryd, so there are no issues with Bluetooth, but GCM doesn't recognize my FR945.
Phone: Samsung S10
OS: Android 10 v4.14
GCM v4.28
FR945 Firmware 3.3
ANT / BLE / BT 5.01