Then there is nothing the OP can do , but I guess he/she could contact the ISP with the same question.
But this OP is puzzled as to why changing the bit of wire makes such a difference when many others have said it shouldn't have that effect.
4 different 5 metre lengths tried, one made with CAT5e cable, plus one 10 metre because someone suggested that 5m might be a magic number that was causing the issue
I have contacted my ISP and they're as puzzled as anyone else but, as pointed out, the cable they supplied is the one giving best results. It's just not long enough to site the router on a safe shelf instead of the end of my desk where I've had to use cable ties to stop it falling off.
Going back over this thread will show that I've tried all sorts of things, removing the faceplate to use the test socket was one of the first.
It looks like I had better tighten up the cable ties as the router will be staying where it is.
EE Fibre Plus