Node.js app working on my local machine but not others

I’m in the process of setting up a node.js app and just got it working on my local machine, but it’s not loading on friends or family’s laptops or my phone.

I followed the instructions in https://www.digitalocean.com/community/tutorials/how-to-set-up-a-node-js-application-for-production-on-ubuntu-18-04.

I have a few suspicions about what might be causing the problem, but I’m very new to this and wanted to see if you all had any suggestions. In descending order of suspiciousness:

• I set up LetsEncrypt. I noticed that when the app fails to load on another computer on Chrome, the URL bar is indicating that the URL isn’t secure, even though I have LetsEncrypt and the website shows up as secure on my machine. During setup, I also indicated option 2: Redirect - Make all requests redirect to secure HTTPS access
• I’m not sure if I’m running Babel properly. I have presets set in my package.json, but not sure if I need to run it manually on the server (this feels wrong, but I don’t know of a better way)
• I’m running node with node -r esm server.js (with ES6 code, arrow functions and the like). I’ve been testing with the newest versions of Chrome so this shouldn’t be a problem, but I’m not sure, esp. if I’m not using Babel properly
• Set up IPv6 on my droplet