You can turn off the mobile network on your iPhone using the airplane mode to avoid roaming and turn on the wifi after to access wifi networks.
You will find all the information here : https://support.apple.com/en-ca/HT204234
You can also take advantage of your mobile plan everywhere you go using the daily traveller pass : http://www.videotron.com/residential/mobile/travel#/tab/cat3700012
Have a great day!
Anne, did you see that there is a new blog post about this topic on the Community ? Check it here maybe it can help you !