With Pi-hole installed on your network, you have to make clients use it for their DNS. There are 3 common approaches:
Edit the router's DHCP settings and enter the Pi-hole's static IP as the DNS server. Now the router's DHCP server will tell clients to use your Pi-hole for DNS.
Switch off the DHCP in your router and enable the DHCP server on your Pi-hole in Settings > DHCP. This may be the better option if your router's DHCP is very limited and does not support editing settings.
Everything continues to use you router for DHCP and DNS as before, and you tell your router itself to use your Pi-hole for DNS. The disadvantage is that on the Pi-hole every request comes from your router (so you cannot see which device is requesting it) and there are multiple DNS hops needed (client -> router -> Pi-hole > Unbound -> possibly external DNS depending on how Unbound is set up).
Method 1 is probably the most popular and flexible, followed by 2 then 3.