Because your router isn't a DNS server

All nodes on the Internet communicate with each other using IP addresses, not domain names. Nobody wants to remember the IP address of every single website that they want to visit, so DNS was invented to associate "friendly names" that people could remember, to IP addresses. DNS is the phone book that maps domain names to IP addresses.

When you type www.flyertalk.com, or any other URL, into your browser, your PC asks its DNS server for the IP address of www.flyertalk.com. DNS responds with the IP address, then your PC communicates directly to that IP address.

So, getting back to my first point, your router is not a DNS server so therefore pointing your PC's DNS settings at your router would have no effect and you'd have no name resolution. Your ISP should maintain a DNS server that you can use.