You can't normally get a wireless connection from the phone socket to the router.
The normal system is to plug your router into your phone socket (the main one is better than an extension) and then to have wireless connection from your router to the computer. Remember that your router must be plugged into a microfilter, which then plugs into the phone socket.
If I understand you correctly, you have quite a long lead from your phone socket to the router (20 ft or 4 metres - one or the other!). This seems un-necessarily long, unless you have no alternative.