The currently recommended MTU to connect to Telecom ADSL2+ is 1500. The TD-W8980 will not save an MTU value for PPPoA greater than 1492. I understand that with MSS-clamping, this issue may not affect TCP, but may negatively impact other protocols such as UDP.
Mercutio reported a similar issue with the TD-W8970, and earlier with the TD-W8960; Mercutio reported that TP-LINK did fix the TD-W8960.
I have reported the issue to TP-LINK and their reply includes: "... I confirmed with our R&D person, they claimed that apart from the first 8 PPP digits data, 1492 is the largest MTU we can set. And it is not likely to be able to set it to the value of 1500 in the next firmware ..."
So is TP-LINK implying that the "effective" MTU is 1492+8, ie 1500?
Whatever, it seems one should be reserved about choosing a TP-LINK TD-W8980 to connect to Telecom ADSL2+ until TP-LINK fixes the firmware to allow the Telecom NZ recommended PPPoA MTU to be set. It seems to be a matter that TP-LINK should have been on top of since at least their TD-W8960 model.