Reopening this thread to check in on the status of your OnePlus 9. Are you still encountering sync and login failures?
If so, we recommend installing ARC from the Android APK downloader instead of the through the Play Store. Note you’ll need to manually update each release if you use the APK installer - however, this workaround ought to prevent hanging on the login screen.
Here is the download link for the Android installer: https://download.roonlabs.net/builds/RoonARC.apk