Add the Smart SIM QR code to your phone, using the camera of the phone you want your Smart SIM on. If you've only got one phone to hand, follow this guide to add it manually.
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The Smart SIM can take up to an hour to install on the phone.
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Update your IMSI and Data Roaming settings using this guide.
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If you have a Google Pixel phone, or have turned on Private DNS yourself, this needs to be turned off. Here's a guide on how to do that.
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Check out the guides on setting the Smart SIM as your Primary or Backup SIM