Have you tried updating your Bluetooth connection, as described in the sticky thread in this forum? This may have fixed the issue for me.
I was also having problems with Bluetooth not automatically reconnecting when taking my watch out of range of my smartphone and back into range again.
To reconnect, I have had to open Garmin Devices in Connect Mobile and wait.
Garmin device: Forerunner 630, Software Version 7.10, Bluetooth/ANT 2.40.
Android device: LG Google Nexus 5x, Android 7.1.2 Beta (build NPG47I).
Connect Mobile: version 3.16
The fact that I am running a beta version of Nougat means I'm wasn't totally surprised that there were issues.
Yes...but it asks for a PIN CODE that I don't know
When the phone finds the watch the watch will post a pin code that you need to enter to proceed.
Since I upgrade my android version to Nougat the stability of the bluetooth connection was bad, lots of breaks and disconnections.
After some search found out that was the battery optimization of the Garmin Connect Mobile that was giving those problems and need to disable it.