Hi,
Task Server is not supported outside the premises; you need to assign some existing TS for the Default Internet Site to let the CEM clients work with it. Please try following:
NS Console -> Settings -> Notification Server -> Site Server Settings.
Select some existing Task Server.
In the left pane in "Detailed Information" section, switch "View" to "Internet Sites".
Press "New" -> dialog "Select a Internet site" should appear.
Select desired Internet Site in this dialog and press OK.
After these actions you should be able to see your TS under "Site Servers" node of Internet Site.
Thank you,
Alex.