Matomo open source analytics installed, but can't access it because of 404 error. Simply a question of custom HTML files?

The open source part of Ghost also lead me to seek alternatives to Google Analytics. I landed on Matomo, but in self-hosting it I haven’t been able to make it work.

I tried placing Matomo, within my Ghost server, in two different locations. The first was, /var/www/html (as per the FLOSS Marketing School video on how to install Matomo*). I pointed my browser to cafebuenvivir.com/matomo and got a 404 error.

Alright, I Googled my issue and found this other Ghost forum post*. It suggested placing my ‘custom files’ within the assets folder of my theme. Not ideal, since I would want to retain my ability to change themes, but if it’s the only way to make it work I’ll try. In my attempts, I either moved the files from /var/www/html or downloaded the Matomo files at /var/www/ghost/current/content/themes/casper/assets/. Then I went over to cafebuenvivir.com/assets/matomo, and nothing, 404 error.

My URL is cafebuenvivir.com.
My Ghost version, given by my “ghost version” command is:
Ghost-CLI version: 1.14.1
Ghost version: 3.31.1 (at /var/www/ghost)
I’m hosted at the free tier Amazon Web Service server. Ubuntu 18.04 LTS. I followed the self-hosted install for Ghost instructions.
I’m using Brave browser.
I downloaded the Matomo files using wget (as they themselves recommend in their installation guide), instead of FTP (I’m not skilled at using FTP).

*I’m sorry I’m not linking these. I can only give three links, the rules say, given that I’m a new user. However, I figured if it was important enough, you’d be able to find the content through the key words I gave you. On the other hand, I did give you my personal URLs, since you can’t google those.

**UPDATE: After some tinkering and reading, I think my problem may be an issue with using Nginx as the engine for Matomo. Not that I shouldn’t use Nginx, but that the Github guide to using Matomo through Nginx says I’m probably using php-fpm. Yet I think I’m not using php-fpm. I didn’t even have it installed. This means, I guess, that Ghost processes PHP requests in a different way than the usual Nginx Matomo installation. Given this, do you know how I could get Nginx to use the same resources as Ghost so that they can both run in my measly machine?