The declaration and everything is in the header file, error from my side to write .cpp in the gist. Sorry about that!
Also I should mention that in the .cpp is only a no-op like GetStructFromBox,
void UVHServerCommunication::JsonToStruct(const UProperty* structToFill, const FString& json) {
check(0);
}
The StructBoxLibrary is only in beta so maybe that is something that does not compile as well? I will try to set that up and see if StructBoxLibrary does work with nativizeAssets.