What are the connection stats, from the modem/router, when it's plugged into the master test socket?
Have you checked the obvious (there's a micro-filter in every single 'phone socket that has something plugged in)?
Maybe remove the ring wire (3) and the spare wire (4) if they're connected in the master, and the extension socket(s).
If it's a very long line, then problems on the internal wiring could push it over the edge.
The simplest solution would be to buy an ADSL face-plate/splitter, fit it to the master socket, and connect the modem there. If the modem (or router) cannot go next to the master socket, wire a CAT5 cable to extend it.
Ade
Giving UKFSN/Entanet a try
DL Sync 4600+ (now 2464 and screwed up by pointless 15db SNR target)
UL Sync 448