Likely cause of ADSL not working on extension

I'm going to investigate a problem with a friend's broadband connection
tonight, but thought it would be worth sounding out the collective
wisdom here on what the likely causes of the fault were.

The friends moved house recently and signed up with Sky for their broadband.
I installed the modem/router for them on the house's one phone socket and
it all worked, using the sky-supplied filter.

They really wanted the router elsewhere, as the place where the phone
socket was is somewhat inconvenient. Yesterday, they got someone to
install a telephone extension and moved the router + filters to that
extension. Their internet connection hasn't worked since. I've checked
with them that both phone sockets are working and that the router
still powers up. I'm going round there to investigate further.

Possible causes I can think of are:

(1) They haven't got a filter on both the sockets, or else they've
connected the router to the filter incorrectly.

(2) The router has failed in some way after being moved

(3) There's something wrong with the way the extension has been
wired up.

(1) I can check easily. (2) I can also check, by moving it back
to the original socket, and also by connecting to its local management
interface. (3) I'm not so sure about. If I rule out the other causes,
then there must be something wrong with the way the extension has
been wired in, but it's not clear to me what to look for, nor
what to tell them needs changing.

Any thoughts ?
.

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