You need to get back to the makers of these products. You are asking why and not for any workaround.
Screenshare requires all devices know how to work with each other. And setup may not be something intuitive. That is, call LG now and ask if these 2 things do work together with Screenshare and if not, go ahead and ask why.
I just got an lg v20 and ive had an lg 55" 4k smart tv since 2015..on my old phone(an lg g5)the screenshare feature worked flawlessly..on the v20 it will not work..i dont understand why??please help