Any ideas??? Tech ping.
Use an Ethernet cable to plug the computer directly into the DSL modem. Run a speedtest and if you get the advertised speed then the router is the problem. If not it may be the DSL modem or, possibly, the computer.
The beest first test is the direction connection from computer to DSL modem.
Sounds like you are going to be calling the phone company. When you are on the support call, have them clear the modem from they server's registration. (First, you have to turn yours off.) They may not know what you are talking about. If that is the case, ask to move up to Tier Level 2 support in the US. They will probably fight that issue but insist. The Tier 2 tech should know what to do. If he doesn't, insist he get help from his supervisor.
They should also be checking the integrity of your line. Tier Level 1 should be able to clean your line if there are problems. Again, if they don't know what you are asking for, ask for Tier 2.
If they can't clear up the line, ask for them to physically change which line your DSL line ports through from CO to your the box at the street. They will fight this issue beyond hell freezing over. Just insist that you are not getting the service you are paying for. Do note, that you may be thinking you are paying for 2.5 mbs but that isn't what you are necessarily contracting are for. That may be the maximum they will provide and not the minimum that they must provide.
Have fun!