Interesting info here. I have an ECI modem, so that shouldn't cause any issues.
I run Linux on most of my machines, just the kids on Windows.

After some recent speed tests I'm quite happy with the results, so things have settled down nicely.

Checking there is a dial tone on the landline and that the profile is what it should be should be a basic requirement.

If installed by a proper Openreach engineer, and not a contractor, then an end to end pair quality test, and a five minute VDSL test are mandatory, this all followed by an 'Eclipse' test using the punters own phone line.
If the above had been done on the first visit, you'd not have had the issue.