Why I just can’t update the OS version on cellular data

At present, an iPhone can only be updated over cellular data IF you have an iPhone 12 or an iPhone 13 with 5G Service.

Meanwhile, for your iPhone 11, make sure that your iPhone is setup to meet the following requirements before you attempt to update the iOS version over a Wi-Fi Network. Open Settings ➡️ General ➡️ Software Update: If there is an update available for your iPhone then it will be listed here. Follow the on-screen instructions to download and update your iPhone. 

• Your iPhone must have a fast Wi-Fi connection; you cannot update over cellular data or a cellular hotspot
• The iPhone can not be in Low Power mode
• The battery state of charge must be above 50%
• Your iPhone must have at least 6GB or more free storage space
• Your iPhone cannot be connected through VPN
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).

Axel F.

ZAKI—— Said:

"I’m not able to update the OS software version on cellular data"

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See my User Tip for Reference: Use an iPhone 12 or later, that is using 5G, in order to update over Cellular - User Tip