I understand that this is a very specific part in the process of setting up a reverse proxy on a Cpanel/WHM server but I hope that it still helps you achieve your goals, even if it is just overcoming one hurdle. Below are the steps that I discussed in the video for you to set up the part of this not included in the video.
Enable Proxy Modules in Apache
Go to Software, and then EasyApache 4 inside WHM and we’re going to back up our configurations first before proceeding.
Click on Convert to Profile under Currently Installed Packages, name and save it and download the JSON configuration file so you have a backup to restore if anything was to go wrong.
Click on Customize and then click on Apache Modules. Search for proxy, change Page Size from 10 to All, and now enable the following:
Continue to click Next until arriving at the final screen. Click Provision.
Best of luck to you in your journey of setting up a reverse proxy!