Are you able to retrieve any line stats? Line interference can cause DLM to reduce your speed for instance. Has anything changed in your house since line speed dropped? New equipment, phones, cables?
I had my speed drop about a year ago and I believe I found the issue and rectified it, though it could be a coincidence I can't be certain. I had a long power cable for the modem I installed and ravelled it up to make it neater. My speeds dropped but I put this down to my partner putting Xmas lights near to the router. Could still be that that was the issue, not sure but, going back to the power cable for the router, I tuned a radio into 612kHz and found it to be making a noise near to it. I unravelled the power lead and the noise went away. Several weeks later my speed came back. Then sods law, I had a phone fault the day after, then multiple disconnects causes by the BT engineer made the speed drop again.
There are so many factors than can affect your line speed. Even someone hundreds of meters away, if they have some dodgy equipment and it cross your line along the way that can cause issues.
https://kitz.co.uk/adsl/rein.htm