Retrieve old text messages
How do I retrieve text messages that were saved Forever and not deleted?
Thanks
iPhone 6, iOS 12
If texts are included in your backups, restore a backup. Remember that when you restore a backup, you overwrite the current contents of your device.
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
If you sync messages with iCloud and they have been deleted from the device, they are gone. iCloud does not store your text messages. It just keeps them in sync across devices.