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

Screen rotation is a must on devices like ... to the "Quick Settings" menu (Windows key + A) and toggling the Rotation lock ...

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 ...