Please go to WordPress admin -> Menus -> Manage Locations to re-assign your menu.
It’s a default WordPress behavior, when you switch themes, the menu will be dropped.
After for slider only showing shortcode text, I believe you are seeing the layerslider shortcode after you have done demo content import.
You are seeing this shortcode because layerslider is not included in our theme.
My employer has not updated the demo content, he will be updating it and include it in next theme version update.
Please see this documentation https://theme4press.com/docs/setting-up-the-sliders/ for the available sliders.