You dont want the clients to get two DNS servers assigned as the router DNS service isn't Pi-holed and the clients will query both.
From screenshot, this appears to be an Asus router.
Me and this other piholer experienced we cant use our Asus router to assign DNS servers to clients via the routers DHCP because we cant get ride of the second DNS IP (the router).
So we switched DHCP from router to Pi-hole.