Can't access shadow root

[Lars-jnk]

I try to access a button behind a shadow root but I get the error:
org.openqa.selenium.TimeoutException: Expected condition failed: waiting for by id or name "btn" (first) (Lazy Element) (tried for 5 second(s) with 500 milliseconds interval)
I try with:

@Unshadow(css = {"main-layout", "#shadow-root", "#myBtn"})
 private FluentWebElement btn;

I also tried several other selectors and combinations. But I'm not sure what path I should go. The documentation is not very deep and should be extended.
Here my dom:

I use the latest io.fluentlenium version.