A big problem somewhere on your line.
Ignore the estimates form other ISPs, they all come from one or other of three (or so) BT databases. These vary between being wildly inaccurate, and auto-adjusting over time to the actual being achieved. You seem to be seeing the latter effect.
The attenuation on your line indicates a potential connection speed of well over 3.5Mbps. Possibly over 5Mbps. The higher than usual downstream noise margin perhaps indicates a long-running instability, and is costing you around 1Mbps.
Please can you get the stats with the filter plugged into the test socket, on the wall at the back in this pic
, without the phone connected to the filter. In fact, preferably with the phone(s?) unplugged from mains power as well if they are that kind.
Have you recently changed anything like the phones or the router? TV, anything?