mobile billing

My wife and I have been on the Xfinity unlimited mobile plan for 6 years. We drove across the border into Canada for 3 days and returned with a $1700 bill from Xfinity. We have the global travel plan and have had no issue being charged and paying the $10 global travel plan daily rate when we are overseas or out of the country. Have no idea why or how we were hit with such excessive roaming charges. Please advise the best way to resolve. We will not be paying this bill and if not resolved will be cancelling our Xfinity mobile plan this week.