Why can’t I use my own data to upgrade my phone

Yes, you can but it depends on the current iOS version that you are updating from.

If you are updating the iOS after skipping many iOS releases then this option may not be available to you..

Didn't you get the option as shown below when you started the update?

You need a phone capable of using a 5G network to update using cellular.

Your iPhone X is not capable of this. 5G is only available on iPhone 12 and newer.

You will either need to update using WiFi or your computer.

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support