A solution to this problem is to configure it via bootstrap/app.php:

use IlluminateFoundationApplication;
use IlluminateFoundationConfigurationExceptions;
use IlluminateFoundationConfigurationMiddleware;
use IlluminateSupportFacadesRoute;


return Application::configure(basePath: dirname(__DIR__))
    ->withRouting(
        commands: __DIR__.'/../routes/console.php',
        using: function (){
            $adminPort = (int)env('ADMIN_PORT',8334);
            $routerFile = (int)request()->getPort() === $adminPort?'routes/admin.php':'routes/web.php';

            Route::middleware('web')
                ->group(base_path($routerFile));
        },
    )
    ->withMiddleware(function (Middleware $middleware) {
        //
    })
    ->withExceptions(function (Exceptions $exceptions) {
        //
    })->create();

As you can see I am using ADMIN_PORT from .env:

ADMIN_PORT=8443

And upon:

$routerFile = (int)request()->getPort() === $adminPort?'routes/admin.php':'routes/web.php';

As you can see i removed the default config upon withRouting and I am using callback in order to resolve it.

The url is the same but now the application required 2 ports to served with:

• Normal http port (80/443) for the routes configed upon routes/web.php
• The once configed upon ADMIN_PORT with default being 8443 and will resolve routed configed upon routes/admin.php

Webserver config

If using nginx, a reccomended vhost configuration is:

server {
        listen 80 default;
        server_name  _;

        return 301 https://$host$request_uri;
    }

    server {
        listen 443 ssl;
        listen 8443 ssl;
        server_name _;

        root /var/www/html/public;
        index index.php;

        ssl_certificate /etc/nginx/ssl/www.crt;
        ssl_certificate_key /etc/nginx/ssl/www.key;

        location / {
            try_files $uri $uri/ /index.php?$query_string;
        }

        location ~ .php$ {
            include   fastcgi_params;

            fastcgi_param PATH_INFO  $fastcgi_path_info;
            fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
            fastcgi_param SCRIPT_NAME $fastcgi_script_name;

            fastcgi_pass php_app:9000;
            
            fastcgi_buffer_size 128k;
            fastcgi_buffers 4 256k;
            fastcgi_busy_buffers_size 256k;
            fastcgi_read_timeout 300s;
        }
    }

At config above you can set server_name with your domain if multiple domains used on a single server. Also check if default is needed as well in your case.

Pay attention that for the same vhost I listen both into 443 and 8443:

listen 443 ssl;
listen 8443 ssl;

Thus The same application for the same domain will be served upoin same url defined in server_name. Also keep in mind to config the keys accorditly as well.