DNS leak. hsd1.il.comcast.net shows up as a search domain

I am trying to manually set my own DNS servers in System Preferences. However, it seems like the router is not honoring my computer's DNS servers and instead is using some sort of Comcast DNS server. (I am using https://dnsleaktest.com/ to check.) I think this is because hsd1.il.comcast.net is showing up as a name server although I am not exactly sure. 

Does anyone know how I can stop the router from ignoring my computer's list of DNS servers?