The following should hopefully solve your problem: start sleep as android wait for ~15 minutes click on menu > backup > upload to cloud. The upload should be unrestricted at that point. Let us know if that does not help.
Please first check to you have sufficient connectivity, ideally a home WiFi at the time of synchronization. If synchronization is still an issue, please send us a debug report directly from the backup add-on in Settings > Backup > Menu > Send debug.