Missing Personal Hotspot on my iPhone 13 Pro Max after iOS 18.1.1 update

Well, you should upgrade it to 18.3, then contact your carrier about the personal hotspot. And, Do you have a 3rd party VPN installed, even if you are not using it? If so, delete the VPN profile in Settings/General/VPN & Device Management/VPN. Don't just turn it off; delete the profile. If that fixes it, you can try reinstalling VPN.

“Personal Hotspot options disappears in Iphone 13 pro max soon after my device upgraded to 18.1.1.”

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Updating an iPhone:

When upgrading the iOS version, should have installed the latest-available iOS: iOS18.3. Did that not install?