I have followed everything in your post and still can't change IP.
I am with Comcast in cental Illinois - using their business class service. They say that the IP is dynamic, not static.
I am using a Lynksys router. I have tried the MAC clone, but it doesn't help.
I have comcast business as well but my IP is static. However, a friend of ours also has comcast business with a dynamic IP and his IP hasn't changed in over a year.
Since you've tried cloning your MAC with no success, that means the IP is being assigned via the cable modem. I don't think there's any way to change the info on that device. So no matter what MAC you put after the cable modem, comcast sees the same modem and assigns the ip accordingly.....hence the ip not changing.