Switched to my own modem was switched off of unlimited plan?

I worked with XFinity support to lower my bill, they suggested that I could save money by purchasing my own modem. I drove out and bought the recommended modem, then waited an afternoon for a service tech to "swap out" the XFinity modem for my new one.

However, I was just notified that I am close to exceeding my bandwidth limit?

I had been on an unlimited plan - it seems that it was turned off after I started using my own modem. I spoke to support, and they told me that it would cost me an additional $30 a month for unlimited data - but, if I rented one of their modems it would only cost me $15.

So, spending a day to setup a modem, which I was told would save me money, actually ends up costing me more.

I continued to talk to the support rep, and they told me that the overage fee was only a one time fee of $10. Which is actually cheaper than paying the unlimited plan! Why would anybody pay $15 a month when the overage fee is only $10 a month?

I cannot express how frustrating it is to try and work with Xfinity - it seems that they have specific training to make the customer experience as frustrating as possible.