Thanks for reaching out to the Apple Support Communities. I understand you'd like to restore some deleted messages, and I have some information that may help.
To restore those messages, you'll need to restore a backup from when those messages were present. Doing so will restore your iPhone to the state it was in when the backup was made, so if you have added any important data since then, you'll want to copy it manually before restoring.
Have a look at the article below to learn how to restore from a backup.