ctkDateRangeWidget just does not have a designer plugin. To fix it, two files has to be added to CTK (in CTK\Libs\Widgets\Plugins) and add the class to CMakeLists.txt and ctkWidgetsPlugins.h. It would be great if you could submit a pull request with this change to CTK. You may not even need to build it, we can test it before we merge the pull request.
In the meantime, as a workaround, you can add a frame in Qt designer and then put the ctkDateRangeWidget in the frame in your Python code.