Vivoactive will not stay synced to Garmin connect mobile

Given as a present. Worked brilliantly for first 7 days, synced and received notifications. Phone battery ran low and watch played up. Somehow my profile settings were changed ie changed my age and sex. Reset both back to original settings. Now the watch will not stay synced to the Lumia 535 windows 10 phone unless the Garmin mobile app is open on the phone screen. Still unable to receive notifications.

I have paired the watch through the mobile app and still does not work. Have checked its the latest software on both devices but nothing seems to fix this. Welcome any advice.