How to setup CUSTOM DNS in a DUMB AP

So, I have recently created a DUMB AP using my old router Archer C20 V04. But it just doesn't work when I try to look up guides for setting up CUSTOM DNS from various sources, it just doesn't work. And most of them are related to the routers that work as Main APs. So, I would like to get some good guides to set up Custom DNS in a DUMB AP.

Configure upstream DNS provider, but for LAN interface.

I can only see Custom DNS fields and not any other like "Peer DNS" , "IPv6 DNS".
Edit: dnsmasq, firewall and dhcpopd are enabled in startup just for you info I'm sharing this information as I have seen dnsmasq is related to this thing some how.

Ignore that part if your LAN interface is configured with just a static IPv4 address.

it didnt do anything. No internet. Only few sites like youtube, amazon are working. Actually my apartments main router/AP has DNS problems. So, the internet is working properly only if I use Cloudflare WARP service from each and every device. But I am tired of connecting to Cloudflare WARP everytime I boot my PC and use my phone. So, I wanted to setup Custom DNS so that I can overcome this issue without contacting the ISP as my Apartment's owner is reluctant to fix that Router/Main AP.
Edit : Internet is working while using Cloudflare WARP when I use internet through my Dumb AP and also Main AP too

You can setup IP/DNS manually in your clients. That's probably easier than reconfiguring the router. If you go for the latter you might want to set up a NAT network to separate the dhcp servers cleanly.

Your statement is convincing enough to not to waste my time for now.

Sounds like dumb AP is not suitable for you, but resetting OpenWrt to factory defaults should work.

Actually, I am gonna live in this apartment temporarily. So, I don't need a separate internet connection for me. But the Apartment's Wi-Fi doesn't have enough range to reach my room. So, yea here I am struggling with my DUMB AP and DUMB Owner. LOL