Flat twin telephone extensions will give you noise problems. This is what is causing your reduction in speed and noise margin. If you must use an extension, then get one made up of twisted pair cable, such as that sold by Run-it-Dircect They will make one up to your specifications.

Alternately, get a long RJ11 cable to connect your router to the master socket. Provided this is of good quality you can go up to about 15 m with little reduction in your speed. Run-it- Direct could also make up one of these to the length you require.