Sorry to hear that you are having an issue activating that second set top box. Sounds like you have upgraded your account to not only adding a second set top box, but a new internet plan, and that internet plan is not supported by your current modem.
The order for all of that is written together, and they are unable to upgrade the services seperately. Also they don't want you to be subscribed to a higher internet speed than what your equipment can handle, and thus be paying for something that you can not get.
Now you should have been offered to have one shipped to you, or to pick one up at your nearest store. I hope that clarifies what is going on, and that you are able to take advatage of the higher internet speeds with that new modem.
If you have any further questions, there are other customers here that can help. I will too. I answer questions on my time off, as I am a customer also.