iPhone can't back up to iCloud

My iPhone 6 running iOS 9.0.2 cannot back up to iCloud. The problem has been going on since I had iOS 8.4.1 though.

When I go to Settings > iCloud > Backup, it's set to on. But when I select Back Up Now, it just says "The last backup could not be completed. Last Backup: 9/16/15 at 9:56 PM." Possibly relevant, I was traveling overseas on this date, but using my Phone (I have an international plan).

To address this problem, I have

• In Settings > iCloud > Backup, toggled iCloud Backup off and on.
• In Settings > iCloud > Storage > Manage Storage > Backups > iPhone This iPhone > Delete Backup tried to delete the file (which is 0 KB), but get the error message "Cannot Delete Backup. This backup cannot be deleted at this time."
• On my MacBook Pro's iTunes > iPhone changed Automatically Back Up from iCloud to iTunes, then backed up to iTunes, they changed the setting back to iCloud
• Shut off the iPhone and turned it back on
• Did a soft reset of the iPhone by holding down the power and home buttons until the Apple logo appear.
• Contacted an Apple Care specialist who has turned the case over to a technical expert (I think that's the person's title).
• Tried deleting the backup file via iCloud.com, but neither the specialist nor I were able to locate the file.

None of those efforts have helped.

Any suggestions? Thanks!