There could be several reasons why you are still seeing ads while you are browsing.
A few things to check:
Make sure your clients are using the Pi-hole IP (as the sole DNS) server.
If your network is set for IPV6 IPs then make sure that the clients use the IPV6 IP of the Pi-hole and not the router (or anything else).
If you don't care about IPV6, you can disable it front he router all the way but keep in mind that if you do that, if a website has an IPV6 address only, you won't be able to access it.
See if analyzing/fixing these issues, resolves your problem.