This is also occuring on my Archer A7 router.  I just installed this router and am running a pi-hole, but after renewing the IP, both my pihole IP and the router IP showed up as a DNS server.  Kinda defeats the purpose of the pi-hole if it doesn't respond fast enough and is bypassed.  Just thought I'd let you know it's affecting more than one model of router.

@phireph0x 

Nothing yet, I will reach out and see if there is more that can be found and let you know

@Witchboard 

That is kind of a different situation as Decos actually have a 3rd DNS server that is not currently selectable, standard routers do not.  I would recommend starting a thread on your model specifcally.  

Thanks.  Will do.

@phireph0x it actually very annoying

On default settings of router mode connected behind a Huwaei GPON router for fibre, my devices gets 2 dns servers one is the main router gateway and second is deco gateway which causes internet instability on devices.

phireph0x wrote

I've manually configured DNS servers for my LAN clients through the Deco DHCP Server, using Cloudflare's 1.1.1.1 and 1.0.0.1.  However, the DHCP server continues to pass my gateway address as a third DNS server.  How do I prevent this?

is this similar to what u have been describing.

@mfritzen i believe this thread is not more maintained and I am also waiting for a response from them.

In access point mode will it again pick up the 3rd DNS servers apart from the default gateway.