We're sorry to hear of your app connection issue! Please see the link below to find more information about how to setup your CarBack with the app. If you need additional support contact us at ridersupport@trekbikes.com , attn: Brian.
https://www.trekbikes.com/us/en_US/carback-radar-rear-bike-light-setup/
Ride Bikes, Have Fun, Feel Good!