This device like all remote controllers are not designed to be used with the existing light switch… it is designed to replace the existing switch. Just don’t order the hand held remote but the in-wall remote that replaces the wall switch function for you. You can see it in operation here in my review.
Or you can use the in-wall and the hand-held if you desire to have two controllers not to mention of course voice control which is the way @Stephan and I do it as well 