Feels like your DHCP server is not giving the address of your pihole as the DNS to the devices. Did you change the DNS settings on your router/DHCP server? Or disable the router/DHCP and enable pihole's DHCP server?
You can check by running ipconfig /all (on a Windows CMD prompt) and checking the DHCP and DNS info for what your expect.
Here's the guide for pihole DHCP: How do I use Pi-hole's built in DHCP server (and why would I want to)?