Connection type is PPPoE or PPPoA? If it is PPPoE, go to Network---WAN Settings to Edit and change it to PPPoA. Maybe it helps. Give a try and post the result here. I take the hint from posts here....

exactly same issue with other product TD-VG3631 fw version: 0.6.0 1.1 v0001.0 Build 150505 Rel.39381n also isp is Telstra