On Unlimited plan, but Mexico data roaming charges still applied

Hello - I was on a short visit to Mexico and despite the Unlimited plan that I am on having "Unlimited talk, text, and data in Canada and Mexico" (see https://www.xfinity.com/mobile/learn/plan) was still charged several hundred dollars of data roaming charges. Can you please look into what that is the case, and help in waiving off these charges that should not be there. thanks..