I want to use cellular date to update my iPhone

You can update using cellular data IF you have an iPhone 12/13/14 AND you have a 5G connection. You have an iPhone 11 and do not have 5G capability with that iPhone. You will, therefore, have to update over a Wi-Fi Network.

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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
• Ensure that you do not have a beta profile installed from a previous version
• 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.

If you still have the iPhone 11, you cannot use cellular to update the phone.

Use Wi-Fi....or....connect the phone to your Mac or PC.