two tpcs running on the same port, nginx and digitalocean - node.js

Question

I am deploying my react app on digital ocean. I have followed this tutorial and everything went just fine: https://www.youtube.com/watch?v=lN0oiYqenpA&ab_channel=RyanMichaelHirst
Now whenever I go to my ip or domain name, I the following error: 404 not found.
whenever i run the log of nginx it says that the port 80 is being used. therefore i run this:
sudo netstat -pan | grep ":80"
tcp 0 0 159.203.87.191:80 52.84.150.39:31164 SYN_RECV -
tcp 0 0 159.203.87.191:80 52.84.150.39:38386 SYN_RECV -
I know that, that is an internal error, but I can't seem to find the error on that.
I have two things running on the same port and everything is crashing. It was working yesterday and I was getting the welcome to nginx page.
This is my server config.(nginx)
upstream my_nodejs_upstream {
# (this is the server's ip address)
server x.x.x.x.;
keepalive 64;
}
server {
listen 80;
listen [::]:80 ipv6only=on;
# 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 mmt-university;
location / {
# First attempt to serve request as file, then
# as directory, then fall back to displaying a 404.
try_files $uri $uri/ =404;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_pass http://universitymmt.com/;
proxy_set_header Host $http_host;
proxy_set_header X-NginX-Proxy true;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_redirect off;
proxy_read_timeout 240s;
}
# pass PHP scripts to FastCGI server
#
#location ~ \.php$ {
# include snippets/fastcgi-php.conf;
#
# # With php-fpm (or other unix sockets):
# fastcgi_pass unix:/var/run/php/php7.4-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;
#}
}

Answer 1

The connection is in SYN_RECV state because the kernel has received a SYN packet for a port that is in LISTENING mode, but the other end did not reply with ACK.
Check if the ACK is received by the server by running the capture on the server. Is the capture taken on the client or on the server?
Also, a considerable number of SYN-RECV, indicates a possible SYN Flood Attack performed against the host. For more info:
https://www.cloudflare.com/learning/ddos/syn-flood-ddos-attack/