How do I hard reset iPhone 11 Pro with volume buttons?

You're likely delaying the actions too long.

Follow this exactly:

1. Press and quickly release the volume up button
2. Then immediately press and release the volume down button
3. Then immediately press and keep holding the side power button and KEEP HOLDING it past the slide to power off and KEEP HOLDING it until you see the Apple Logo, then let go

You have to do the actions quickly, until you hold the sold power button, which you KEEP HOLDING.

You are describing what is known as a Forced Restart.

Quickly press and release the Up button

Quickly press and release the Down button

Hold the Side button for 20-30 seconds while you ignore the slider and anything else until the Apple logo appears in the center of the screen

Then.....release the Side button to let the iPhone restart

A Forced Restart will always work.....unless......the phone has hardware problem.

In that case, the only option is to make an appointment at the Apple Store or take the phone to an authorized Apple service shop.

You can find a list of authorized shops in your area using this link: Find Locations