I have a samsung s7. I hope this may useful information for some.
I connected my device with the 'trusted device' feature. It maintains a rock-solid, reliable connection.
How I know. I made a mistake.
I am on my second edge 830.
When pairing (setup) my device-to-phone for the first time with the first device, I did not select the the trusted device option. I subsequently had to unlock my phone and open garmin connect to get BT to work correctly. It worked, but seemed like too many steps.
Then my second device arrived as a replacement. This time I selected the trusted device option. My connection established each and everytime, whether or not I unlocked my phone.
Then I made the mistake of synching my new 830 device in garmin express to the 1st edge 830 device profile. My ability to connect took me back to where I had to unlock the phone and open garmin connect first.
So, I unpaired my phone and went through the pairing setup again to 'again' select 'trusted device. I also removed the first device from garmin express.
For me, using trusted device provides a rock-solid, reliable connection that only requires me to power up my garmin edge 830.
Hope someone can get some use my lesson.