Install the Flex eSIM on the day you depart. That timing keeps the QR handy while making sure the profile is fresh when you arrive. Installation requires an internet connection (Wi-Fi or your home SIM data), so complete the steps while you still have a reliable connection.
Once the eSIM is added, you remain in control of when it activates. Simply toggle the Flex line on and enable Data Roaming after you land and it will attach to the local network automatically.
