If you're not using Pi-hole as DHCP, then the Pi-hole's IP address needs to be added under the LAN DNS settings in your router. If your router does not allow you to set this, then disable the DHCP server on your router and enable it on Pi-hole. Once this is done, you may need to disconnect your clients from the network and reconnect them for them to get addresses from the Pi-hole DHCP server.
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