Deleted messages

If you're using iCloud Syncing for messages, once you delete the messages, they are gone.

If you're not using iCloud Syncing and you have a back up that contains the messages, you can restore your phone from that back up:

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

Note that doing so will overwrite everything currently on the phone.

Read here https://discussions.apple.com/thread/250472221?answerId=250901093022