Data Usage issues

I recently signed up with a new plan on the 4K flex Box and a 600MB they're megabytes I just received my 1st notification that I am close to going over my data limit.  I was previously a customer with Comcast for over 20 years and had to take a break so I could find a new place to live and I never received any notifications regarding data usage please advise how I can get rid of this issue issue I have gotten a new gotten a new gateway modem them but that doesn't seem to be helping