Automatically restart

Remove the SIM, memory card and insert again

This is a manufacturing defect of exynos 9611 soc and a quality control fault of Samsung
They had provided free repair to many users but now it has become widespread
https://r2.community.samsung.com/t5/Tech-Talk/Auto-Restart-Brand-solution/td-p/10333335
Follow the above steps in the post, if they deny, then ask for discount and be adamant that the update has damaged your phone, tell them you will file complaints in consumer forum

Dear Samsung member,

Greetings from Samsung customer support.

We acknowledge your query and apologize for the inconvenience caused to you and will surely assist you regarding the same and please follow the easy troubleshooting steps which might help to solve the issue.

There are two methods to disable the auto-restart function
Method 1: From the home screen swipe up or down to access the App Screen > Setting > General Management > Reset > Auto Restart > To Turn OFF auto-restart slide the slider toward the left.
Method 2: Go to 'Settings'>>Tap on 'Battery and device care'>> Tap on Three dots>>Tap on 'Automation'>>Disable 'Auto-restart at set times' if enabled

If you are facing this issue after installing any new application then uninstall that application.
Navigate to the Apps screen and tap on Settings>>Tap on Apps>>Tap on the desired app>>Tap on Uninstall>>Tap on OK

Reset settings: From the home screen, click on Settings>>General Management>> click on reset and then reset setting.

If the issue still persists we would request you to kindly visit your nearby service center, for the physical inspection of your device. Please be assured, our team will surely help you with this.

You can search for the nearby service center from the given link:-https://www.samsung.com/in/support/service-center/

Thank you for writing to us.
Warm Regards,
Samsung Customer Support.