Recently bought my own modem and after returning the xfinity provided modem I no longer have Unlimited Data?

I bought my own modem under the impression that it would save me the $11-14 rental fee on my bill for using Xfinity's provided modem/router. Now, i'm getting an email stating that I'm almost out of data and it's $30 to add unlimited? So it's nearly double the cost of the rental fee to retain unlimited and not use their equipmement? Can anyone make this make sense? I was never told I would lose unlimited and feel like I've been tricked into paying more for less. When I call customer support i'm taken on a loop of canned mumbo jumbo and they tell me my only option is to add unlimited to my ever growing bill. The fact that it's more expensive to use my own equipment and it's not clearly stated that I would lose unlimited seems extremely shady.