Please note that the below configuration is going to use php-fpm for what is worth.

/etc/nginx/nginx.conf

# For more information on configuration, see:
#   * Official English Documentation: https://nginx.org/en/docs/
#   * Official Russian Documentation: https://nginx.org/ru/docs/

user  nginx;
worker_processes  1;

error_log  /var/log/nginx/error.log;
#error_log  /var/log/nginx/error.log  notice;
#error_log  /var/log/nginx/error.log  info;

pid        /run/nginx.pid;


events {
    worker_connections  1024;
}


http {
    include       /etc/nginx/mime.types;
    default_type  application/octet-stream;

    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  /var/log/nginx/access.log  main;

    sendfile        on;
    #tcp_nopush     on;

    #keepalive_timeout  0;
    keepalive_timeout  65;

    #gzip  on;

    # Load modular configuration files from the /etc/nginx/conf.d directory.
    # See https://nginx.org/en/docs/ngx_core_module.html#include
    # for more information.
    include /etc/nginx/conf.d/*.conf;

    index   index.html index.htm;

    #root /usr/share/nginx/html;

  # this is the section that matters for AppMail.
    server {
     listen      80;
     server_name domain.com;
     root        /usr/share/nginx/html;

     location / {
         if (!-e $request_filename){
             rewrite ^(/)?api/.*$ /api/index.php;
         }
         if (!-e $request_filename){
             rewrite ^(/)?customer/.*$ /customer/index.php;
         }
         if (!-e $request_filename){
             rewrite ^(/)?backend/.*$ /backend/index.php;
         }
         if (!-e $request_filename){
             rewrite ^(.*)$ /index.php;
         }
         index  index.html index.htm index.php;
     }

     #error_page  404              /404.html;

     # redirect server error pages to the static page /50x.html
     #
     error_page   500 502 503 504  /50x.html;

     # pass the PHP scripts to FastCGI server listening on 127.0.0.1:9000
     #
     location ~ \.php$ {
        include /etc/nginx/fastcgi_params;
        fastcgi_pass  127.0.0.1:9000;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;

        fastcgi_read_timeout 600s;
        fastcgi_send_timeout 600s;
    }

     # deny access to .htaccess files, if Apache's document root
     # concurs with nginx's one
     #
     location ~ /\.ht {
         deny  all;
     }
   }
}