You can do it like this:

server {
    listen 80;
    listen [::]:80;

    root /var/www/html/public_html;

    server_name example.com;
    if ($http_x_forwarded_proto = 'http'){
        return 301 https://$host$request_uri;
    }

    # landing page
    location =/ {
        # try to serve file directly, fallback to index.php
        try_files $uri /portal.php$is_args$args;
    }

    # other pages
    location / {
       rewrite ^(.*)$ /portal.php?id=$1 last
    }

    # LEGACY
    # This rule should only be placed on your development environment
    # In production, don't include this and don't deploy app_dev.php or config.php
    location ~ ^/(portal)\.php(/|$) {
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
        fastcgi_pass unix:/run/php/php7.2-fpm.sock;
        fastcgi_split_path_info ^(.+\.php)(/.*)$;
        fastcgi_param HTTPS on;
        # When you are using symlinks to link the document root to the
        # current version of your application, you should pass the real
        # application path instead of the path to the symlink to PHP
        # FPM.
        # Otherwise, PHP's OPcache may not properly detect changes to
        # your PHP files (see https://github.com/zendtech/ZendOptimizerPlus/issues/126
        # for more information).
        fastcgi_param DOCUMENT_ROOT $realpath_root;
        fastcgi_buffer_size 128k;
        fastcgi_buffers 4 256k;
        fastcgi_busy_buffers_size 256k;
       internal;
    }    

    # return 404 for all other php files not matching the front controller
    # this prevents access to other php files you don't want to be accessible.
    location ~ \.php$ {
        return 404;
    }

    error_log /var/log/nginx/project_error.log;
    access_log /var/log/nginx/project_access.log;
}

This way it works:

location / { 
    try_files $uri $uri/ @rules; 
} 

location @rules { 
rewrite ^(.*)$ /portal.php?id=$1 last;
}