I stopped supporting iOS 9 in October 2022 after Apple stopped supporting it in its developer tools, so anyone still using BandHelper on iOS 9 is using a 3 1/2 year old app version. Now, the connection code used in that app version is incompatible with the current SSL certificate installed on the sync server, so that app version can no longer sync.
I've been hearing from users who are still using that old app version on iOS 9, and the best solution is to upgrade to a newer device that can use the current app version. The app currently requires iOS 15.
There is one workaround that might reenable syncing on iOS 9. I no longer have a device with iOS 9 to test it with, so I can't help you set it up. Also, running an old app version on an unsupported device will eventually lead to other problems. But if you want to try it, you can manually install the attached certificate chain. You'll need to find instructions for this on your own. A Google search for "manually install certificate on iOS 9" had some good info, but I don't see one single source for complete instructions.