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You are configuring an SSL cert for the custom domain you want to use (E.g. app.mycustomdomain.com), not the *.mydevices.com URL that we provide to you. E.g. app.mycustomdomain.com.
The cert can either be requested for free in AWS ACM, or a commercial SSL cert from GoDaddy or other SSL cert provider can be purchased and imported to ACM. We recommend using ACM free certs because it's free and it handles renewals automatically.
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