Lantiq ADSL2 AnnexM

I have a TP-Link TD-W8980 V1 router running 17.01.5 and everything runs pretty smooth until I change the ADSL mode in /etc/config/network through LuCI and restart the router to make it change the mode. It is not working on AnnexM.

Although the router runs smoothly with ADSL2+ it just does not sync with AnnexM and keeps showing idle-request, handshake and exception in the ADSL status view. I am not sure whether the issue is with firmware itself or is it my line. The ISP has confirmed that my line has been upgraded to AnnexM. Any pointers here?