İsternesiz hosting'ten proxy yapmak yerine Amazon'dan proxy yapabilirsiniz.

server {
    listen 443 ssl http2;
    listen [::]:443 ssl http2;

    server_name example.com;
    root /var/www/html;

    access_log /var/log/nginx/example.com.access.log;
    error_log /var/log/nginx/example.com.error.log warn;

    location / {
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
        proxy_set_header X-Forwarded-Host   $host;
        proxy_set_header X-Forwarded-Port   $server_port;
        proxy_set_header Host   $host;
        proxy_http_version 1.1;
        proxy_cache_bypass  $http_upgrade;
        proxy_set_header Connection "upgrade";
        proxy_connect_timeout  300;
        proxy_pass http://hostingip;
    }
    location ^~ /forum {
        root /var/www/html;
        try_files $uri $uri/ /forum/index.php?$query_string;

        location ~ \.php$ {
            try_files $uri /forum/index.php;
            fastcgi_pass unix:/var/run/php5-fpm.sock;
            include fastcgi_params;
            fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        }
        location /forum/admin {
            try_files $uri $uri/ /forum/admin.php?$query_string;
        }

        location /forum/api {
            try_files $uri $uri/ /forum/api.php?$query_string;
        }
    }
    ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
}
server {
    listen 80;
    listen [::]:80;

    server_name example.com www.example.com;

    location / {
        return 301 https://example.com$request_uri;
    }
}

Flarum subdirectory nginx ayarlarını https://serverfault.com/questions/75...-laravel-nginx buradan aldım.
Yuakrıdaki config ile example.com/flarum ile girenler /var/www/html üzerinden unix:/var/run/php5-fpm.sock ile kullanılıyor. Diğer yerler içinse "hostingip" ile reverse-proxy kullalınıyor.