SOLUTION!!
I just moved over to the Sprint network and had this same issue; I was finally able to update Sprint Carrier settings from 28.2 to 28.4!
My exact steps:
Close iTunes, disconnect device.
Download the latest carrier settings .IPCC file-
I used https://ipcc.cf/ and specifically downloaded the version 28.4 file for iPhone on sprint from https://ipcc.cf/carriers/us/sprint/ (saved the file to my desktop)
Open a terminal on your iMac or MacBook (applications>utilities>terminal)
Copy and paste the following line:
defaults write com.apple.iTunes carrier-testing -bool YES
Press enter. Reattach your device.
Open iTunes. (The same error as before- "update carrier settings", try to download and error as before).
NOW FOR THE FIX- choose the device summary page (the page with the device name, backups, options, etc).
HOLD DOWN the OPTION key and click "check for update".
A window will open (rather than connect to the internet), then navigate to your desktop (to wherever you downloaded the .IPCC file), and choose the downloaded .IPCC file.
The update is done instantly! I then checked the carrier settings and they were updated to Sprint 28.4 , I turned off my phone and turned it back on, reconnected to iTunes, and no more error! The carrier settings stayed at 28.4!
You're welcome!