So here is the thing. If DNS is not set it will use immediate parent's DNS.
To go in more details,
If in your computer DNS is not set then it will take router's DNS. --> This will show router's IP as DNS.
Now if in Router DNS is not set , then it will take ISP's DNS. --> This will show ISP's IP as DNS.
Unless you manually specify, it won't show the DNS. If all the automatic, computer will show router's IP as DNS, Router will show ISP's IP as DNS.