Select the IP option from the firewall menu and click on (+).
Go to the general tab and select input from the Chain drop-down menu.
From the protocol menu, select UDP, and in the "Dst. Port input" box, put the value 53.
From the" In. Interface" sub-menu, select the desired WAN Interface.
Go to the Action tab, and select the Drop option.
Click on Apply and Ok options.
By selecting (+) again, select the Input option from the Chain menu
Select TCP from the Protocol tab menu and Set the "Dst. Port input "box to 53.
Go to the" In. Interface "tab and click WAN Interface.
Click on Apply and Ok.