This app was mentioned in 8 comments, with an average of 1.25 upvotes
It's software 9.0. It's happening to everyone. Set watch to stand alone (Phone: off in the settings). Delete watch from Garmin software on phone. Delete from bluetooth on phone (unpair). Then reconnect using Garmin software on phone..
I've been using this: https://play.google.com/store/apps/details?id=mabi.android.gcmrunner&hl=en_US
I had a similar problem about a year ago with my 5x+. I found a factory reset of the watch was what fixed it.
Other thoughts:
Are you having Bluetooth connectivity issues with other devices? Like headphones or mice or whatever?
Do you have other devices attached or constantly attached? I sometimes have bluetooth problems when my phone is connected with my computer using the Windows phone app
I also know there is an app (3rd party) called "Reconnect Garmin Watch" https://play.google.com/store/apps/details?id=mabi.android.gcmrunner&hl=fr_CA
I had this problem with a Samsung and an LG phone. I finally downloaded the below free app which has kept the connection working since.
https://play.google.com/store/apps/details?id=mabi.android.gcmrunner
Had this issue with my HTC m8 phone and Fenix 3. There was an Android app to help it reconnect: Reconnect Garmin watch
It worked great for me at the time. I haven't needed it since I upgraded to the HTC 10 a while back.
Try this: https://play.google.com/store/apps/details?id=mabi.android.gcmrunner for reconnecting the phone. If you can't install it on your phone, then ask the developer to allow it for you phone. I have done so yesterday and it is solving my BT reconnection problem on Huawei P9 Android 7.
I've a Fenix3, and an HTC One M8 phone. Watch and phone won't stay connected, which is apparently a known (but denied to be known) issue. On the forums, found an Android app that polls and reconnects them periodically. Despite its name, I think it will try to reconnect any Bluetooth device. May be worth a shot.
https://play.google.com/store/apps/details?id=mabi.android.gcmrunner&hl=en_US
It's the 9.0 update software (atleast on Fenix 5 plus). Use this.
Try this app it solved my problem. https://play.google.com/store/apps/details?id=mabi.android.gcmrunner