It sounds like your modem isn't able to pick up the NBN_VDSL signal on your phone line. This can be for a variety of reasons, but most often is due to a physical one such as a fault with a socket, phone cable or the wholesale cabling between your house and NBN’s equipment.
Some simple troubleshooting might be able to resolve the issue,
1) Turn the modem off for 20 seconds and turn it back on again. After a minute or two, see if the DSL light goes solid and if so, try your connection again.
2) If the DSL light still flashes or stays off, please check that the phone cable between your VDSL modem DSL port and the phone socket is firmly plugged in. If it is, please try another phone cable, as these can often develop faults.
3) If you have more than 1 phone socket in your house, have you tested your modem at your other sockets? If there us anything plugged into your other sockets, eg alarm, fax, phones please remove these from the line and retest.
4) Unstable power supply can also cause issues for DSL modems. If your modem is plugged into a power board, please try it directly into a wall socket and see if there's any difference.
If you are able to take photos of your setup and lights on the modem and attach this would be a great help in identifying your connection issue.