Nginx upstream how to proxy?

Question

Nginx

Hi all.
Set up nginx balancer (nginx upstream). That's just it does not proxy, but simply redirects to the server. Is this his job? Or I set it up incorrectly .... Please help me figure it out.
I understand that Balancer should proxy to other servers and not change the domain name, but it changes to subdomains.

Nginx balancer conf:

upstream backend  {
        least_conn;
        server s2.domain.com:8082 max_fails=3 fail_timeout=100s;
        server s1.domain.com:8081 max_fails=3 fail_timeout=100s;
        }

 server_name _;

        listen 443;

        ssl on;
        ssl_certificate /var/www/ssl/site/*.domain.com.crt;
        ssl_certificate_key /var/www/ssl/site/*.domain.com.key;

        ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
        ssl_ciphers 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA';
        ssl_session_cache   shared:SSL:10m;
        ssl_session_timeout 5m;
        ssl_prefer_server_ciphers on;
        ssl_stapling on;
        ssl_stapling_verify on;
        add_header Strict-Transport-Security 'max-age=604800';
        #length_hiding on;
        add_header Content-Security-Policy-Report-Only "default-src https:; script-src https: 'unsafe-eval' 'unsafe-inline'; style-src https: 'unsafe-inline'; img-src https: data:; font-src https: data:; report-uri /csp-report";
        ssl_dhparam /var/www/ssl/site/dhparam.pem;


        location / {
       proxy_read_timeout 1200;
       proxy_connect_timeout 1200;
       proxy_pass http://backend/;
       proxy_set_header Host domain.com;
       proxy_set_header X-Real-IP $remote_addr;
       proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
       proxy_set_header HTTPS on;
        }

Nginx client 1 conf:

server {
        listen 8081;
        server_name s1.domain.com;
        root /var/www/site/public/public/;
       #https://s1.domain.com$request_uri? permanent;
        }


server {
    listen   8081;
    listen 443 ssl;
    server_name s1.domain.com;
    root /var/www/site/public/public/;

    index index.php index.html index.htm;

        ssl_certificate /var/www/ssl/site/*.domain.com.crt;
        ssl_certificate_key /var/www/ssl/site/*.domain.com.key;


        ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
        ssl_ciphers 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA';
        ssl_session_cache   shared:SSL:10m;
        ssl_session_timeout 5m;
        ssl_prefer_server_ciphers on;
        ssl_stapling on;
        ssl_stapling_verify on;
        add_header Strict-Transport-Security 'max-age=604800';
        #length_hiding on;
        add_header Content-Security-Policy-Report-Only "default-src https:; script-src https: 'unsafe-eval' 'unsafe-inline'; style-src https: 'unsafe-inline'; img-src https: data:; font-src https: data:; report-uri /csp-report";
        ssl_dhparam /var/www/ssl/site/dhparam.pem;

sendfile off;

    client_max_body_size 100m;


    location ~ \.php$ {
#        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass unix:/run/php/php7.0-fpm.sock;
        fastcgi_index index.php;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_intercept_errors off;
        fastcgi_buffer_size 16k;
        fastcgi_buffers 4 16k;
        fastcgi_read_timeout 60s;

#Fastcgi cache config
#       fastcgi_hide_header "Set-Cookie";
        add_header X-Cache $upstream_cache_status;
        fastcgi_cache SITE;
        fastcgi_cache_bypass $no_cache;
        fastcgi_no_cache $no_cache;

    }

    location ~ /\.ht {
        deny all;
    }

}

Nginx client 2 conf:

server {
        listen 8082;
        server_name s2.domain.com;
        root /var/www/site/public/public/;
        #rewrite ^(.+)$ https://s2.domain.com$request_uri? permanent;
        }


server {
    listen   8082;
    listen 443 ssl;
    server_name s2.domain.com;
    root /var/www/site/public/public/;

    index index.php index.html index.htm;

        ssl_certificate /var/www/ssl/stage/*.domain.com.crt;
        ssl_certificate_key /var/www/ssl/stage/*.domain.com.key;


        ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
        ssl_ciphers 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA';
        ssl_session_cache   shared:SSL:10m;
        ssl_session_timeout 5m;
        ssl_prefer_server_ciphers on;
        ssl_stapling on;
        ssl_stapling_verify on;
        add_header Strict-Transport-Security 'max-age=604800';
        #length_hiding on;
        add_header Content-Security-Policy-Report-Only "default-src https:; script-src https: 'unsafe-eval' 'unsafe-inline'; style-src https: 'unsafe-inline'; img-src https: data:; font-src https: data:; report-uri /csp-report";
        ssl_dhparam /var/www/ssl/site/dhparam.pem;
   sendfile off;

    client_max_body_size 100m;


    location ~ \.php$ {
#        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;
        fastcgi_index index.php;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_intercept_errors off;
        fastcgi_buffer_size 16k;
        fastcgi_buffers 4 16k;

#Fastcgi cache config
#       fastcgi_hide_header "Set-Cookie";
        add_header X-Cache $upstream_cache_status;
        fastcgi_cache SITE;
        fastcgi_cache_bypass $no_cache;
        fastcgi_no_cache $no_cache;

    }

    location ~ /\.ht {
        deny all;
    }

}