The licensing of Felgo and Qt is fully independent. Regardless of the Felgo Plan you are using, it is additionally required to comply with the licensing terms of Qt. This means, that you can either use
the Qt Commercial license or
the Qt Open Source license, which requires to adhere to the LGPL licensing terms.
It is also possible to publish applications in the stores, that you build with Qt Open Source License.
Adhering to LGPL does not mean you have to share your application and make the full source code public in this case. It is sufficient to provide the compiled object code (not your actual source code) upon request, which rarely happens anyway.
You are can choose the Qt license and Felgo plan that best fits your project.