Hey Rayalaashok,
Thanks for posting in Apple Support Communities. We appreciate the details you've provided. If we understand correctly, your iPhone unexpectedly restarts when trying to back up to iCloud over Wi-Fi. We're happy to help.
Which version of iOS is currently installed on your iPhone? To check, go to Settings > General > About. If you have any pending updates, please install them before trying again: Update your iPhone or iPad - Apple Support
Does your iPhone only unexpectedly restart during iCloud backups?
What is your Battery Health percentage? Does your iPhone have performance management applied? To check, go to Settings > Battery > Battery Health & Charging.
Scroll down to the "Battery Health" section of the following support article for more information: iPhone Battery and Performance - Apple Support
You can also create iCloud backups via computer: How to back up your iPhone, iPad, and iPod touch - Apple Support
We look forward to hearing back from you. Take care!