Simple Mobile is a prepaid carrier utilizing the T-Mobile network within the United States. You’re not able to use the Simple Mobile service when roaming in Canada. There is no option or additional service that allows this coverage either. When you go with a pre-paid carrier, you’re going with the bargain brand. This isn’t expected and is typical with prepaid cell phone carriers.
If you need another option besides Simple Mobile if you’re traveling to or from Canada, you should be able to find other prepaid cards in airports and other stops where you enter and exit the country. Roam Mobility is one of them I know of.
Most Simple Mobile phones are unlocked for use in other countries but I would make sure your phone is before traveling. Some Simple Mobile phones would not work in Canada because the phone was actually a T-Mobile branded phone and did not require unlocking to use Simple Mobile.