Couldn't able to Block Ads with Primary and Secondary DNS

[ChipMinder]

I performed a fresh installation of Pi-hole and Unbound DNS using DietPi. I have configured my router to use Pi-hole as the primary DNS, and when I set Cloudflare's DNS (1.1.1.1) as the secondary DNS, I'm not able to block ads. However, when I remove the secondary DNS, Pi-hole is able to block ads successfully.

The issue I am facing:
https://tricorder.pi-hole.net/jtUMArSR/
Details about my system:
DietPi running on NanoPi R6S
What I have changed since installing Pi-hole:
Fresh installation

Thank you

[deHakkelaar]

Have a look at below FAQ:

Why should Pi-hole be my only DNS server?

Reduced Ad Blocking Capability The main reason you should Pi-hole as your only DNS server is that you will see increased performance in the blocking of ads. If you have two DNS servers (Pi-hole and something else), your network clients may not always query Pi-hole for name resolution. If a query happens to be answered from a non-Pi-hole DNS server, your block lists will not apply (since that DNS server doesn't know about them). All Queries Need To Go Through Pi-hole First Since other DNS serv…

That bit about "Pi-hole and something else" could also apply for your router and is most likely what your experiencing.