Thanks John for the response.
I realized that my SSL certificate wasn’t the problem, but that MM wasn’t providing a method for delivering the CA bundle cert that was issued with my certificate.
So I decided to try something a little different. I am now hosting the MM server behind an apache proxy (which I can provide my cert, my key, and my ca-bundle cert). Now the browsers and mobile apps are working properly!
I have posted a detailed explanation of what I did within this support page (that I found thanks to you!!!).
[solved] Mobile App(Android) TLS handshake error - Troubleshooting - Mattermost Discussion Forums