The background sync shouldn't happen at all while you're using the app, so if that's what the problem is, whether you're offline or not doesn't matter.
Do you have App selected in Account > Users > Edit > Notification Types in the BandHelper website?
Also, does Samsung allow you to install a later Android version? Since this post I've seen numerous miscellaneous display issues with Android 7.0, but many device manufacturers don't allow later Android bug fix releases to be installed. Ideally if you have any Android 7 version, it would be 7.1.2.