Browsers often allow zooming of a web page by scrolling the mousewheel and holding down the CTRL button. By default, this will work on sites built with the Divi Theme. However, you may find that it no longer works on your Divi site, without an obvious reason. Here's how to fix it.
Disable "Smooth Scrolling" in the Divi Theme Options
If you can't zoom with the mousewheel, one possible explanation is that the following Divi setting has been enabled:
Divi > Theme Options > General > Smooth Scrolling
Divi says you can "enable this option to get the smooth scrolling effect with mouse wheel". However, in doing so, it seems to disable zooming with the mousewheel in combination with the control key. Disabling the setting should restore the mousewheel zoom. Note, though, that you'll lose the smooth scrolling effect.