@Geoff-1 

Hi, TP-Link DSL modem routers support in 2 working modes, DSL modem router and wireless router. If the DSL modem is not connecting to the internet, you may try the following troubleshooting:
Case One--No internet on DSL modem router mode:
1. Make sure the DSL/phone cable is plugged into the DSL socket on the modem.
2. Check whether there is a valid internet IP address under Advanced > Status. 
    (1) If the IP address is 0.0.0.0, check with the internet service provider whether you have got the correct username and password.
    (2) If the IP address is not 0.0.0.0, go to Advanced > Network > internet > internet connections > Modify and change the DNS to 8.8.8.8/8.8.4.4.
Case Two--No Internet on wireless router mode:
1. Make sure the LAN4/WAN port on the modem is connected to your NBN box or Ethernet jack on the wall.
2. Confirm wireless router mode is selected under Advanced > operation mode.
3. Check whether there is a valid internet IP address under Advanced>Status
    (1) If the IP address is 0.0.0.0, check with the internet service provider about the internet configuration credentials.
    (2) If the IP address is not 0.0.0.0, go to Advanced > Network > internet and change the DNS to be 8.8.8.8/8.8.4.4
Get to know more troubleshooting steps, please refer to No internet service after quick setup