AWS EC2: When i access the / of the website, it works fine, but when I access directly /signin, it returns NOT FOUND

I deployed my first website on AWS EC2(MERN Stack) and it work fine as long as I access it from / path. So if you access this link, it works alright: http://3.124.204.215/ and you can access any other page from here. But if you try to access this path directly: http://3.124.204.215/signin, it returns 404. I can curl localhost:8080/signin, so I assume that the problem is from nginx.

Is there any property for this problem?

THE NGINX CONFIGURATION:

server {
    listen 80;
    listen [::]:80;
    listen 443;
    listen [::]:443;
    root /var/www/amazon-clone/deploy/build;

    location / {
        try_files $uri $uri/ =404;
        proxy_pass http://172.31.40.72:8080;
    proxy_redirect http://172.31.40.72:8080/ /;
    }

    location /api/ {
        proxy_redirect          off;
        proxy_pass_header       Server;
        proxy_set_header        X-Real-IP $remote_addr;
        proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header        X-Scheme $scheme;
        proxy_set_header        Host $http_host;
        proxy_set_header        X-NginX-Proxy true;
        proxy_connect_timeout   5;
        proxy_read_timeout      240;
        proxy_intercept_errors  on;
        proxy_pass              http://172.31.40.72:5000;
    }
}