Hmm, that’s very odd. Are you using any plugins? One thing you could try is to disable all plugins and see if that ‘fixes’ the problem. If it does then you know you have a theme clash…enable the plugins again one by one to isolate the clash.
Also, what are your slider settings under Appearance > Customize > Slider?
How to add banner code depends on where and what kind of banner you want to add to the theme. If you want it in the header then you’ll need to add your banner code to the header.php template. Keep in mind though you must always set up your theme as a child theme before making any template customization so that future updates do not overwrite your local changes.