I have successfully confirmed that my DSL connection is 1/3 of my full speed when plugged into 2 different wall jacks in my apartment. My building manager linked those two wall jacks together, with port A coming from the D-MARK and port B connected to port A.
Port C is not connected to either of those ports, and is receiving the full bandwidth for my DSL connection. When I plug it into A and B, both of them show the same speed.
There are no phones, no modems, no devices of any type on these lines with the exception of the DSL modem.
Port C is in a very inconvenient location to run wires from. Calling my ISP only resulted in "you need to pay for a technician to come out." I know how to hook up and wire phone jacks, I just have no idea where the fault/problem would be here.
What are some troubleshooting techniques I can try to get the other ports working, or to find out where the true problem lies?