It might be worth "crossflashing" the 8980 with the 9980 firmware - the hardware is the same, and I think the 9980 firmware contains more recent Lantiq DSL code. And also unlock it to give proper telnet access - the TP-Link interface is quite limited and provides very little diagnostic information.
My 8970v1 running 9980 firmware, not connected to any line, logs alternating messages of "xDSL Ready!!" and "xDSL Leave Showtime!!". DaleLamar's logs indicated it got slightly further than that.
@DaleLamar - how long is your line and what speed are you expecting? If you have an extremely long line, it might be possible that one device can just manage to establish a connection, while another can't, with nothing being faulty or incorrectly configured.