I have a Fenix 5 and Iphone 6S. as soon as I upgraded ti iOS11 my Fenix would not pair. I resolved it as follows:
Fenix 5 7 to iOS 11 solution
(1) On Fenix 5
- Hold down Light button (1), goto controls menu
- Power off
- Power on
- Hold down (1), goto controls menu
- select green phone icon to disconnect from phone
(2) On iPhone, Garmin connect
- Delete Phenix 5 device
- click on + to add new device
- Select Fenix 5
(3) On Penix 5
- Hold down Up Menu (5)
- Select Settings
- Select Phone
- Select Pair Phone
- Watch produce a pair code
(4) On iPhone, Garmin connect
- Add pair code to phone
- sync with watch (this takes a while, be patient)
(5) All done!!