I had the exact same problem with my NBN connection, the 4 blue lights on the NBN box were on but I had no internet connection. After a number of calls to TPG and an appointment with an NBN technician (who failed to turn up), and even a complaint to the Ombudsman, in desperation I actually reset the tplink modem to factory defaults and BOOM !!! it worked.
Hope this helps all you others having the same problem.