First, here's an example showing the footer options where they normally appear:
If the "Footer" settings are missing, it is likely your footer is built using the theme builder. This theme builder footer replaces the standard Divi footer and in the process makes the footer options in the customizer, which target the standard footer, irrelevant. In recognition of this Divi hides the footer options from the customizer, but the way in which it does so leaves the footer section briefly visible while the page loads. Once the page is fully loaded the "footer" section is hidden and the section below ("button") moves up to fill the gap.
In order to make changes to the footer you need to instead go into the theme builder ("WP Dashboard > Divi > Theme Builder"), locate the relevant footer and make the changes to it there.