Unable to connect to secure websocket in DigitalOcean - express

Question

I'm having trouble setting up my WebSocket server in Digital Ocean.
I'm changing my actual domain name for domain.com for the question's sake.
I basically have a NodeJs WebSocket server that I'm trying to connect to a react app I'm hosting at Heroku. I'm getting the following error when attempting to connect:
WebSocket connection to 'wss://domain.com/' failed: Error during WebSocket handshake: Unexpected response code: 200
Here's my server entry code:
const PORT = process.env.PORT || 8080
const privateKey = fs.readFileSync('/etc/letsencrypt/live/domain.com/privkey.pem', 'utf-8')
const certificate = fs.readFileSync('/etc/letsencrypt/live/domain.com/cert.pem', 'utf-8')
const credentials = { key: privateKey, cert: certificate }
const server = express()
const httpsServer = https.createServer(credentials, server)
httpsServer.listen(PORT)
this.wss = new WebSocket.Server({ server: httpsServer })
I used cert-bot to secure my connection, as for Heroku is obligatory. So here's my nginx default config file, located at /etc/nginx/sites-available/default
server {
# SSL configuration
#
# listen 443 ssl default_server;
# listen [::]:443 ssl default_server;
#
# Note: You should disable gzip for SSL traffic.
# See: https://bugs.debian.org/773332
#
# Read up on ssl_ciphers to ensure a secure configuration.
# See: https://bugs.debian.org/765782
#
# Self signed certs generated by the ssl-cert package
# Don't use them in a production server!
#
# include snippets/snakeoil.conf;
root /var/www/html;
# Add index.php to the list if you are using PHP
index index.html index.htm index.nginx-debian.html;
server_name domain.com www.domain.com;
location / {
# First attempt to serve request as file, then
# as directory, then fall back to displaying a 404.
try_files $uri $uri/ =404;
}
# pass PHP scripts to FastCGI server
#
#location ~ \.php$ {
# include snippets/fastcgi-php.conf;
#
# # With php-fpm (or other unix sockets):
# fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;
# # With php-cgi (or other tcp sockets):
# fastcgi_pass 127.0.0.1:9000;
#}
# deny access to .htaccess files, if Apache's document root
# concurs with nginx's one
#
#location ~ /\.ht {
# deny all;
#}
listen [::]:443 ssl ipv6only=on; # managed by Certbot
listen 443 ssl; # managed by Certbot
ssl_certificate /etc/letsencrypt/live/domain.com/fullchain.pem; # managed by Certbot
ssl_certificate_key /etc/letsencrypt/live/domain.com/privkey.pem; # managed by Certbot
include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
}
I also changed a bit my UFW config. Here's the output to sudo ufw status
Nginx Full ALLOW Anywhere
22/tcp ALLOW Anywhere
Nginx Full (v6) ALLOW Anywhere (v6)
22/tcp (v6) ALLOW Anywhere (v6)
Just to be clear, I'm NOT using domain.com for real. I just changed it in the current question for privacy concerns, :D
Hope anyone can point me in the right direction. Not really sure where I'm going wrong.