Why low upstream sync rate?

[#1]

Firstly, this isn't a Plusnet problem (Demon ISP), just looking for generic advice.
Market 1 20CN exchange, downstream attenuation 41 dB, SNR~12 dB, average PPPoA uptime ~100 hours, router is an old Draytek Vigor2600 plus series annex A Firmware Version: 2.5.9_UK
Downstream sync rate is 4256 kbps
Upstream sync rate was 96 kbps when I arrived, 288 kbps after rebooting the router, 800 kbps 16 months ago (shortly after resetting the line).
The router does not report the upstream attenuation, as far as I can see.  I'm not used to the upstream sync rate being anything other than the maximum allowed value and so I don't know what sort of errors could be causing the current low values.
Is this anything I or the ISP could do something about?

[#2]

It is unusual for the upstream to be low.
Have you tried a different router ?

[#3]

Wired or wireless?

[#4]

Quote from: MisterW

It is unusual for the upstream to be low.

I don't know, I have not paid much attention to this on previous visits.  And no, I have not tried another router.  It's a wired connection; this router model does not have wireless.