also what are you trying to run that its blocking? if you want a webserver then you'll just have to use a different port like 81 and at the end of the url put :81 ie
http://x.x.x.x:81 if its an ftp then just try random ports like 81 or 444 and so on.
Im not sure about the whole cable modem blocking the ports, first time I've heard that but I know the cable modem controlls your bandwidth speed which I found out a awhile back *cough*