Codeigniter 3x Nginx Config for subdirectory

gistfile1.txt

server {

  listen 8081;
  listen [::]:8081;

  server_name _;
  root /var/www/main_directory;

  add_header X-Frame-Options "SAMEORIGIN";
  add_header X-XSS-Protection "1; mode=block";
  add_header X-Content-Type-Options "nosniff";

  index index.php index.html index.htm;

  charset utf-8;

  location / {
       try_files $uri $uri/ /index.php$is_args$args;
  }

  location = /favicon.ico { access_log off; log_not_found off; }
  location = /robots.txt  { access_log off; log_not_found off; }

  error_page 404 /index.php;

  location ~ \.php$ {
      try_files $uri /index.php =404;
      fastcgi_split_path_info ^(.+\.php)(/.+)\$;
      fastcgi_pass unix:/run/php/php7.4-fpm.sock;
      fastcgi_index index.php;
      fastcgi_buffers 16 16k;
      fastcgi_buffer_size 32k;
      fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
      fastcgi_read_timeout 600;
      include fastcgi_params;
  }

  location ~ /\.(?!well-known).* {
      deny all;
  }

  location ^~ /sub {
      alias /var/www/sub_directory;

  #        access_log {access_log_file{;
  #        error_log {errors_log_file};

      #if (!-e $request_filename) { rewrite ^ /subfolder/index.php last; }

      try_files $uri /sub/index.php;

      location ~ \.php$ {
          try_files $uri /index.php =404;
          include fastcgi_params;
          fastcgi_index index.php;
          fastcgi_param SCRIPT_FILENAME $request_filename;
          fastcgi_split_path_info ^(.+\.php)(/.+)\$;
          fastcgi_pass unix:/run/php/php7.4-fpm.sock;
          fastcgi_buffers 16 16k;
          fastcgi_buffer_size 32k;
          fastcgi_read_timeout 600;
      }

      location ~ /\.ht { deny all; }

      location ~ ^/(application|system|tests)/ { return 403; }
  }

}