Maybe your previous isp was routing to a closer exchange. Have a look at the link below and if you enter your phone number into the top left side of the page it should give you an idea how far away frok the exchange you are.
You might want to try setting up your old adsl modem and see what speeds you get. There is a possibility that your new modem is faulty however you wont know that until you fire up your old device. You could always opt for plan b and speak with support as they may just send you a new modem.