This fully working tracker has a new battery installed but comes with functionality limitations.
It was sitting in a box and missed the opportunity to be upgraded to the latest firmware, so now it will remain on the 6.46 version partially supported by the Fitbit app:
Mobile Fitbit app (iPhone and Android) - data sync and upgrade are not working even after linking it to the app.
The only reason to connect it to the Mobile Fitbit app is to set up/sync time on your tracker.
Don't try to update/upgrade it via any app - it will fail, and there is no workaround.
What works:
What doesn't work: