Same problem here: forerunner 235, iphone 6s on ios 11.0.1. Syncing is not possible. I tried to repair but now my phone and 235 does not find each other.
I have same problem on Garmin Fenix 3 with iOS 11.0.1. It will connect, if you swipe up to Control Center, turn off Bluetooth, then turn it back on, then go to Garmin Connect. It has to be done every time.