Please repost this here »
/fo ··· sedirect and include your modem CM MAC so I can pull account data.
Correcttion after re-reading your post: The specific speed you are referring to is no longer available and once it's dropped it's gone and there is no way to get it back I'm sorry. The reason your modem needs to be replaced is the next tier up is a docsis 3 product and by the sound of it you have a docsis 2 modem.
I will get to the bottom of it however and get it worked out.