Infact I am facing the same problem here with the PPPOE Connection..

I have TL-WR740N v4 .. upgraded to latest firmware released on 15th Jan 2015

changelog to the firmware says pppoe problems solved. ..

but I can't seem to find that solved .. as I am having the same trouble like before ..

I can connect to internet by directly plug it to the PC .. but if I plug to my TL-WR740N .. it connects for sometimes then disconnects .. then never connect again .. Sometimes Cloning Mac does the trick .. but again after a day or two it disconnects & Never connect again.
Tried factory reset, downgrading/upgrading firmware, Rebooting several Times .. nothing works ..
Moreover detecting the connection shows PPOE earlier & after sometime when it starts disconnecting it shows "STATIC" ..
which I find very very strange.
But again I put it back to my PC .. its CONNECTS !!

Checked it in my friend's house who is using same connection .. and had the same problem.
their connection runs fine on D-link or ASUS .. and their D-link worked great on my connection for like a week.

Is TP-Link is really gonna take care of this issue ?? or should I change my network devices priorities to D-link or ASUS.