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That problem has been reported by several users of this model of Garmin device.
One even said it failed to pair after 3 months, also, so there may be some common factors at work.
In any case, one user of the Edge, not the Vivoactive said that uninstalling the Garmin Connect app on the iPhone, and reinstalling the Garmin Connect app again - without un-pairing the Garmin device - got it working again.
Check all your Bluetooth settings on the iPhone, esp. the
on your iPhone, should list the Garmin Connect app as having permission to use the Bluetooth facility.
Also check that both devices, Vivoactive and phone, have Bluetooth turned On, for sure a necessity, and not always to be taken for granted.
Let me know more and I will advise you further. Thanks.