Unfortunately, those libraries are not binary-compatible, so you can’t just copy them over. If you really need the 5.15 version, you can wait for the updated Felgo version in Qt Marketplace (a build for 5.15 is coming soon).
To get some learning, which things are currently broken for you with WebSockets (we use them on a regular base without any issues so far)?
Qt 5.15 in Cloud Builds will be available as soon Felgo officially supports this Qt version. We also provide custom builds for customers within Cloud Builds, if that is an option for you to move on, please contact us at https://felgo.com/contact
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