First, any page that should appear as a link in a subnav, should be added to the menu system.
Once you have at least one page added to the menu system for your subnav, then you can add the subnav (menu block) to your landing page (flexible layout content type).
Navigate to the landing page that you would like to add the subnav to. Click on the Layout tab.
Click on + Add Block.
Click on Create Custom Block.
Click on Menu Block.
Give your menu block any title. This title will not display.
Scroll down to Subnav Block and select Main Navigation.
Click on Add Block button.
You will see a new block with the top level nav items listed. Hover to top right of this block until edit icon appears. Click on Configure.
Scroll down to Menu Levels. Click on arrow to expand. Check Expand all menu links.
Click on arrow to expand Advanced Options. Select the Fixed parent item -- the parent menu item to all the links in your subnav.
Click on arrow to expand HTML and style options. Replace the word "main" in the textbox with "subnav".
Click on Update button.
