The downstream is capped at 55Mb.
The Max Attainable varies between the 2 sets of stats posted so the line should achieve somewhere between 57-59Mb without the cap.
The upstream may also be capped, it certainly looks like it.
The upstream should get the full 20Mb but it's capped at 17Mb.
Both downstream/upstream have Retransmission High.
With the up/down banding this suggests there has been an issue in the lines past. The upstream has had thousands of LEFTERS at some point.
Banding/capping has a tendency to be "sticky".
DLM should remove it fairly promptly if the line remains stable but we see it frequently remain for months (years on some lines).
Banding is usually caused by many resyncs in a short period of time. Testing new modems and those resyncing the line over and over unaware of the workings of DLM are most vulnerable to banding.
What is the ISP? Some ISP's can now request a DLM reset on FTTC lines.
I would be attempting to get this done.
I also advise running
DslStats on the line. This will catch any issues causing bursts of errors or multiple resyncs.
It also gives graphs of the lines Hlog/QLN/Bitloading which would all be great to attach to the thread for us to see and look for physical issues on the line.