It is being used ISP DNS even though i set up my Pi4 router with a different DNS

I am checking what DNS I am using in https://nordvpn.com/dns-leak-test/ (I don't use any VPN, it is irrelevant) but test gives me my country's DNS server. Even though it is like that, I can still enter sites which government DNS blocked sites. Also when I change the DNS in OS networkmanager with the same DNS server that i set on my openwrt router, OpenWRT adblocker doesn't work. Could someone tell me what is going on?

Sounds to me youre missing https://openwrt.org/docs/guide-user/firewall/fw3_configurations/intercept_dns

I did what it says. I am still seeing my ISP's DNS.

Seeing <> using.

Map, say google.com, to 0.0.0.0 on your router.
Try accessing it on your devices, where you think it's leaking.

If it's intercepted, the access will obviously fail.

Keep mind NordVPN will tell you you're leaking DNS queries, just to make you think you need their worthless VPN.