is this an isp provided modem/router? or one you bought?
the isp should be able to help if they gave it to you. if you bought it, then make a nice cup of tea and sit back and do some light reading of the manual (more than likely a digital version from the website). without witting in front of it myself, i can't really offer any insight into how your particular one works. post the model number and such here and when i get some spare time i'll be happy to skim the manual and see what it says about the speed settings but it might take me a day to get it read.
i just got home from a long vacation

and am a bit backed up in my office.