To rotate the screen with the Settings app ... the "Windows key + A" keyboard shortcut and clicking the "Rotation lock" ...

If it's colored, click the button to disable the rotation lock, allowing the screen to rotate freely. If the button is ...

What your iPhone can't do is lock in landscape mode; it can only have its screen rotation locked in the up-and-down portrait view. This isn't so with the iPad, however; iPad screens can be locked ...