Log into self-serve, go to Overview. Mobile services in self-serve from drop-down menu. Report your lost or stolen phone to select. Then select “suspend my phone.” It will lock your account from use. You could try find-my-phone depending on if your phone’s OEM has this option available and also you saet it up if needed. Then you order a new sim card and replace your phone and you opt to reactivate and swap your sim. Here’s more info for you: https://www.koodomobile.com/en/help/reporting-your-lost-or-stolen-phone