After opening the LumenRT in setting bar you could see the option of "Clipping plane setting".
After clicking on the "+" sign you would get an option weather you want to clip your model from Above, Below, North or any other direction.
Clipping Planes - This displays all clipping planes. The switch icon toggles the plane on or off. Clicking the title activates the clipping plane gizmo adjustment. When you add a clipping plane, it displays on this screen. Clicking on the clipping plane name or the arrow displays the color selector so that you can adjust the color of the visible clipping plane.
Clipping Plane Gizmo Control - When the clipping plane gizmo is activated, the plane will appear in a transparent color which is adjustable using the color palette widget. To move the plane, drag the horizontal arrows with the mouse. To rotate the plane, drag along the curved arrows.
Clipping Plane Layers - Clipping planes can be enabled or disabled by layer. By default, all layers are affected by the clipping plane. By clicking individual layers in the clipping layer panel, these layers will be omitted from clipping. This is useful when creating scenarios that need to show cutaways inside buildings.