Restore backup on messages

First, make sure that no new backups will occur. In other words, there cannot be another backup that occurred after you deleted the messages.

Next, let's check to make sure that the Messages really were being backed up to iCloud as follows

Home Screen > Settings > Tap Your Name at the top of the screen

Tap iCloud

Scroll down and check to see that Messages is turned "off"

Restore the iPhone using the iCloud backup

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If you have a device backup that contains the messages, and you do not use messages in iCloud, you can factory reset your device to erase it completely. Then, set up the device using your backup. You may regain your messages but you will lose anything new since your last backup.