I have that same problem. Older models had the option to select in System settings for "over Wifi, over Data, over both, download automatically, always ask" etc. I am not around wifi all the time and just want to get it done when it is available instead of waiting until I can be on a wifi connection. I have searched many times through all the settings and have not found the option to allow it.
If your carrier allows downloads over mobile data, go to Settings< Software Update<Turn off Auto Download over Wi-Fi. If the feature is not available, you will need to contact your service provider for additional assistance.
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