Interesting setup. Not everybody can do that. IMHO and limited experience, TP-Link are the best cable modems

You say you are at just under 1 Mile from the exchange but your modem indicates well over a Mile. Can you please have another look at your Downstream Line Attenuation? It was 26.5dB and AFAIK if it is more than 25dB you should not be able to sync at 40mbps. At over 28dB not even 20mbps can be achived. And at over 31dB your are just the same as with ADSL2+
Here is the "rule of thumb" (found out by myself for VDSL2)
Below 22dB up to 80mbps down.
22dB to 25dB up to 40mbps down.
25dB to 28dB up to 20mbps down.
28dB to 31dB up to 10mbps down.
SNR Margin will get you if you try to "push" the numbers.
Please help me fine tune my table

Measure the exact distance on GoogleEarth and take a few more looks at your stats. See if that 26.5dB was not on the high end on that occasion and usually it is nearer 25dB.
Oh, and TX for bothering to answer.