First, do not operate your iPhone after you delete the messages.
Second, if you have backed up your iPhone, follow this. If not, skip this. Connect your iPhone to iTunes, and restore from a latest backup.
Third, use a 3rd-party tool to scan your phone. it would show all found results, including messages.
How to recover iphone text message ?
Coolmuster iPhone SMS Contacts Recovery is your best choice to recover iphone lost contacts messages completely. With this program, the deleted data can be recovered easily.
How to recover lost messages from iPhone
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