I read you only have a router. That's not right and my bet is there is some modem or modem+router combo.
Depending on the gear (always share make, model, firmware versions) there are sometimes a few tests to see what the issue is. But DSL can be troublesome and you are back to the ISP to fix their system.
In the area I live the internet speed is estimated at 17mbps, however I get around 2mbps on a good day. all my devices have the same problem with the wifi, and even when I turn them all off and preform a speed test on my wired PC it never improves, I have changed the micro filter and also the router, even the Ethernet cable and rebooted the router recently and still have no luck.
Before you ask I only have one phone socket in the house with nothing else connected to it other than my router, I have ran out of ideas and Im getting frustrated paying for a service that dose not deliver on what it says it will, please help and thank you.