Enable SMS & MMS Messages
Now, to make sure your iPhone can send and receive regular non-iMessage text messages, you'll likely need to tweak some additional options. Open your Settings app and head to the "Cellular" menu. From here, if you're on iOS 12, tap "Cellular Data Network." If you're on iOS 11, tap "Cellular Data Options," then "Cellular Data Network."
Opening the Cellular Data Network menu on iOS 11.
Next, you'll have to manually enter a few values into the provided fields. So copy the information below exactly as it's typed (leaving the "leave blank" fields blank, obviously), then enter it into the fields on your iPhone.