@Makedonnnn 

Hi,

Case 1:
If no DNS server is configured under Advanced -> Network -> DHCP Server, then the router will declare itself as the DNS server to the clients that are configured to obtain the DNS server address automatically.
In this case the router acts like a man-in-the-middle between the client device and the DNS server that is configured under Advanced -> Network -> Internet. 
If a client device then asks for name resolution of a certain hostname, this request goes to the router and the router will either answer right away provided it already has the associated IP address (for example, 192.168.0.1 in case of tplinkwifi.net) or, if it doesn't know the IP address it "asks" the external DNS server for it and then responds back to the client device.

Case 2:
If another DNS server (other than the router itself) is configured under Advanced -> Network -> DHCP Server, then the router will declare that DNS server's IP address to the clients that are configured to obtain the DNS server address automatically.
So, if we take 8.8.8.8 as example, the client device would then directly ask Google's DNS server "Please tell me the IP address associated to the hostname tplinkwifi.net". But there is no way Google could possibly know the internal LAN IP address of someone's router.
Instead it will return an IP address that TP-Link has globally associated to tplinkwifi.net, which then leads to some web server on the Internet that shows the user a webpage stating someting like "Trying to configure your router?".
At the time of me writing this post the IP address associated to tplinkwifi.net on the Internet was 3.224.42.34.

In my opinion this behavior is as intended and not a bug. But maybe someone else knows better.

  @Makedonnnn 

The purpose of the DNS is solely to convert hostnames into IP addresses. 

All the traffic to and from the Internet still passes through the router and its firewall.

Besides, as long as you are using external DNS servers there isn't really a need to enter their IP address under Advanced -> Network -> DHCP Server. Just enter them under Advanced -> Network -> Internet.

The main purpose of the DNS entry fields in the menu Advanced -> Network -> DHCP Server is for users who have setup their own DNS server within their local network. 
For example, if the router's IP address was 192.168.0.1 and someone had setup a DNS server on some Linux computer with the IP address 192.168.0.20, then he would enter this IP address under Advanced -> Network -> DHCP Server and clients who obtain the DNS server address automatically would then be directed to use 192.168.0.20 as their DNS server.