Hi. After going back to my 6s for a while i decided to give it another try. I did all the updates on the band and the phone. It works now for almost a week without issue.
I was having the same issue after changing iPhones with iOS 12. I had to reset the band and uninstall Samsung Gear app from my phone, then I was able to pair the watch again and everything is working.
You need the latest software for both the phone and the watch.