To prevent this in the future, please change your network settings from Automatic to Manual and then select Koodo.  This will prevent your phone from connecting to a roaming network.  Note that you may have to redo this after you restart your phone.

https://community.koodomobile.com/plans-services-232910/easy-roam-understanding-how-you-will-be-charged-while-traveling-7811296

I have also flagged a rep to look into your accidental roaming

Hi ​@Jovy, we can see you’ve already spoken to an agent and they’ve added a credit for the roaming charge on your latest bill. We’re not seeing other roaming charges on your past few bills, only the April 6 charge, which has been credited. 

To avoid future charges please make sure that the data roaming is off in your phone settings and you are manually connecting to the Koodo network.

For more information on how to prevent accidental roaming charges, please check out this article https://community.koodomobile.com/plans-services-232910/easy-roam-understanding-how-you-will-be-charged-while-traveling-7811296