A longer modem cable will very likely not make any difference at all, I should have stressed that as my earlier post might well be misleading, with the possibility of its being thought of as implying something that was not intended at all.
Decent modem cables are likely to be pretty much of a muchness imho regardless of length, within reason, where short cables are concerned. An important consideration is also presumably the matter of how many signal volts you have coming out of your dsl line. Since I have 64dB downstream attenuation, that’s a whole different world compared to those if you who are lucky enough to live in civilisation and have short copper, or even something called FTTC [!] which I have heard of but never seen.
And also there is the consideration of how much interference you have around your cable - is the modem cable touching a mains cable, hopefully not. If for some reason you can’t put some spacing between a mains cable and your modem cable, not even a small amount of spacing [!] (why not?), then having the two crossing at right angles is making the best out of a very bad situation and running them parallel is a horrible idea, especially if they are parallel and literally touching.
So all those factors - of cable positioning, dsl signal voltage and kinds of sources of in-house interference - all matter a lot, and affect whether details of modem cables’ length and quality will be important or not.