You can add this to the top of your routes.rb file:
get "/" => "custom_controller#custom_action", as: :custom_root_path
which will give you this route:
custom_root_path GET / custom_controller#custom_action
Then, you can just put the contents of your custom landing page in the corresponding view (custom_action.html.erb file) of the custom_controller's custom_action.
For example, if you name your custom controller as landing_pages_controller and the action is index, then the route will be:
get "/" => "landing_pages#index", as: :custom_root_path
Then, you will have a index method in your landing_pages_controller.rb file and will have a app/views/landing_pages/index.html.erb file where you can put the contents of your custom landing page.