How to access GitLab on subdomain accessed thru Apache 2.4 reverse proxy server

I went thru the installation of GitLab CE on Ubuntu 18.04. I logged in using root. Then changed the root login to to something different. The Letsencrypt cert was obtained. I made the necessary changes per the web interface. The GitLab account was there but I was unable to access the account from my server. I tried various Virtural Host set ups and tweaked gitlab.rb.

I decided to properly uninstall every bit of GitLab CE on and start over hoping to get guidance along the way to success.

As mentioned I completed the install of GitLab CE however I was unable to access GitLab CE from/thru the reverse proxy running Apache 2.4.

Should the both on the server and the subdomain be set up first?
What is the reverse proxy on the server side / domain side?
What is the on the subdomain host and the DocumentRoot of the subdomain host?

What should the gitlab.rb file be modified and reconfigured to reflect the application is running as a subdomain behind a reverse proxy?