503 error behind load balancer

My application is behind a digital ocean load balancer, but it seems like cannot found the routes I declared inside a route group :

Route::group(
[
   'domain' => '123.abc.com',
   'middleware' => ['auth', 'throttle:200,1']
],
function() {
   Route::get('/testing', function() {
     dd("I am found");
   });
});

When I navigate to 123.abc.com/testing, an error of 503 Service Unavailable. No server is available to handle this request occur. But as I remove 'domain' => '123.abc.com' from the route group, everything works well.

Another strange thing is, the error will be triggered even I access a route outside of the route group.

Route::get('/123', function() {
  dd("I am outside of route group");
 });

// route group here

Am I do any mistakes here? Also, is there any alternative way to make the route group accessible from "123.abc.com" only?

Extra notes :

1. "123.abc.com" is pointing to the IP address of digital ocean load balancer
2. The route group with "domain" works well if I point "123.abc.com" directly to droplet IP address
3. My TrustProxies.php configurations :

protected $proxies = "*";
protected $headers = Request::HEADER_X_FORWARDED_ALL;