How to enable arrows in landscape keyboard in iOS 11?
This feature was removed by Apple on all known iPhone screen sizes (4.7" (iPhone 6, 6s, 7, 8) as well as 5.5" inch (iPhone 6 Plus, 6s Plus, 7 Plus, 8 Plus).
Unfortunately there is no way to turn it back on in iOS 11.0.
If you miss the arrow keys from iOS 10 please provide feedback to Apple using the feedback form. Apple actually reads these messages. :)
iPhone/iOS Feedback (Apple)
Use a third party keyboard like SwiftKey or Flesky.