My Garmin Fenix 5 is showing that it has synced with my IPhone but no data is being transferred

I recommend to Remove and Add a Device From the Garmin Connect App | Garmin Customer Support, then log a few minutes of steps and HR, and record a test activity, sync, and see whether it helped. If not, then have a look at the Troubleshooting guide for (some) data missing after syncing for more tips.