Everything plugged into the a phone socket must pass through a filter. It is always better plugging the router into the Master socket, preferably using a ADSL Filtered faceplate.
You can check if your extension is stopping your connection by plugging the router into the Master socket, or Test socket if you have one. You do not need a PC connected to it, just watch the lights and they will tell you whether it is syncing with the exchange. If it does connect then it means your extension is not up to the job. It is important to understand that just because a socket works perfectly with a phone it will work with ADSL, this is not the case.