Locking iphone screen in "Landscape" mode.

Is there any way to lock the iPhone (SE 5) in "Landscape" mode? The default is "Portrait," and the only option seems to be locking it in Portrait, or unlocking it from Portrait. Tilting the phone provides a Landscape view, but then any change of the way the phone is being held/placed automatically returns the image to Portrait. I need to lock it while using a particular app -- how can I do that?