Try to restore your iPhone to factory rest, it will wipe everything from your iPhone. Or try safewiper for iOS to erase your data from iPhone without recovery.

Try some files manager maybe helpful

Believe it or not, the deleted data on your iOS device can still be recovered with recovery apps. Dr.Fone for iOS can permanently erase any personal information you want to protect your privacy. Keep your data safety like contacts, messages, photos, videos and more.

youtu.be/OegJWu7bPTM

Works with almost all Apple devices:
iPhone 6S / 6S Plus, iPhone 6 / 6 Plus;
iPhone SE, iPhone 5S/5C/5, iPhone 4S;
iPad Pro, iPad Air, iPad mini 4 and more