You can turn off the “Enable Frustum Culling” property on the tileset. You may also want to set “Culled Screen Space Error” to the same as the “Maximum Screen Space Error” property (16.0 by default) in order to get the same LOD for the tiles that are outside the view frustum.
Note that this will cause more tiles to be loaded and rendered, which will definitely have a significant performance impact. It’s not clear to me why you need to do this, even in a VR application. But if you need to, you can.