If you're using the SlingStudio Capture app for iOS 11, and you have the orientation lock ON, then you may find that your iPhone/iPod touch or your iPad records video in portrait orientation, instead of the correct landscape mode.
The solution is to deactivate the orientation lock (the control that prevents your device's screen from changing orientation). After you turn off the orientation lock, you should find that video is now properly recording in landscape mode.
If that doesn't fix it, then quit the Capture app by double-pressing the Home button, then swiping the Capture app window upwards to quit it. Make sure that the orientation lock is OFF, and then relaunch the Capture app.
This does not occur with devices running iOS versions prior to iOS 11.