IPHONE x screen off black out

Have you already tried to force restart your iPX?

On an iPhone X, iPhone 8, or iPhone 8 Plus: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the Apple logo.

Regards

Giulio

Since your iPX is brand new, I would contact your nearest Apple Store or Authorized Service Provider and have your device inspected.

You can try to contact also Apple Support directly: Contact Apple Support

Regards and best of luck.

Giulio

Solution worked perfectly for me at first attempt. Thanks so much

You're welcome