Lied to about xfinity mobile cost

Long story short, I was already irritated and dealing with xfinity issues and I live in an area where xfinity has a monopoly on internet fast enough for video games...

I was on a call/chat and the rep told me I could save money by singing up for xfinity mobile.  I was VERY explicit about not wanting to add to my bill.  The frustration was from trying to downgrade everything to save money already.  I was told the phone was free, I would save money and it sounded like it was just a mobile addition to what I had.  I have land line, minimal tv, and high level internet.  We received the phone and followed the instructions.  

I was unaware that billing would be separate and from a separate internet account.  I was under the impression that I was just going to save money.  

Come to find out I am saving $10 in internet but spending $33 on mobile.  The mobile xfinity is charging me $17.91 for the free device and $15 a month for 1GB of data and unlimited talk and text.  I was unaware of this for a month because they send the bill to my comcast email.... which I have never used.  Xfinity wifi sends my bill to a private account, why would they sign me up to be billed at an email that they know I don't use?  

I have mint mobile on 4 cell phones and pay $15 a month per phone for 4 GB.... we don't use data that much.  We also buy cheap Moto phones.  Now, my 12yr old has a new iphone SE (I am not an apple person) that costs $400 and been using extra data (over the 1GB, because I was unaware) and spending $200 a month.  

I can take care of the data usage but even then, I am spending $23 above what I wanted.  

I have talked on the phone to both wifi and mobile companies and they refuse to help me.  I can cancel the service but still owe $358 (or $17.91 monthly) for a [Edited: "Language"] phone.

READ THE FINE PRINT and never get so frustrated that you just say, "fine, as long as it doesn't add anything to my bill."