I would suggest that 'Unlimited' has a very clear meaning to the reasonable person and that they are now breaching their contract as it was initially advertised to the public. Assuming it was initially advertised as 'Unlimited' with relation to speed and data.

Of note; Lets say you were in Australia you could take this case straight to the ACCC or the Communications Ombudsman and from experience I would be fairly confident of you getting what you want.