If your ISP actually wants you to change settings on the modem, they should tell you exactly (step by step) what they want you to do.
The only reason I can think of that they "can't" help you is if you are using your own personal modem that is not one of their approved/supported models. If that is the case, the best thing to do would be to get a modem that works on their network. Otherwise, post the exact model name and number of your modem and we should be able to help you make the necessary change.