So after doing a lot of digging, I found out that the error was that NGINX needs to be configured for the SSL. This isn't really mentioned in the Ubuntu nodeBB guide, and the NGINX configuration guide linked on it is a dead link.
First, I needed to combine my certificate and intermediate certificate (If you have a root certificate, you combine it as well) into one .cert file. This amounted to just copying the intermediate certificate and pasting it after the regular certificate (keeping the "BEGIN" and "END" lines).
Then, I inserted the combined key into /etc/ssl/ directly. After doing this, I added my private key to a new folder I created in etc named "keys" (/etc/keys/).
Lastly, I referenced my SSL in the "default" file in /etc/nginx/sites-available/default
listen 443 ssl default_server;
And restarted nginx via
sudo systemctl reload nginx