I would check with AT&T again. I have a Go phone plan and an AT&T hot spot that I use in the US.
The following is the text from AT&T that shows it works in Canada with some restrictions at no additional costs for talk and data.
Quote
nternational
Includes unlimited nationwide calls and messaging in and between the U.S., Mexico, and Canada. Also includes use of the high speed data allotment from your plan while in Mexico or Canada. Data usage exceeding the high-speed allotment will result in reduced data speeds of up to 128 Kbps for the rest of your 30-day term. Number of texts and number of voice minutes while in Mexico and Canada cannot exceed 50% of the total no. of texts or voice minutes you use in a month, for any consecutive three month period. Also includes unlimited messaging from the U.S., Mexico, or Canada to over 100 countries. To make international long distance calls from the U.S. to countries not included in this plan - click here for details. Coverage and svc. not avail. everywhere. Limited availability in select markets outside of the U.S.
Unquote
The go phone is a pay by the month plan, I only use it when in the US over the winter and put some money in again the next winter, otherwise I don't use it. It's been great in the US, 4GLTE service in most places.