Using classes with different namespaces in Laravel 8

I am trying to use a library of classes within Laravel 8. I am having difficulty getting the classes to load correctly. I created a new folder within the App folder called DigiSigner, which is the external library's namespace.

App\DigiSigner
  DigiSignerClient.php
  \libs
    BaseRequest.php
    Branding.php
    ClassLoader.php
    Config.php
    Curler.php
    DigiSignerException.php
    DigiSignerResponse.php
    Document.php
    DocumentField.php
    DocumentFields.php
    ExistingField.php
    ExportObject.php
    Field.php
    SignatureRequest.php
    Signer.php

I created a controller that looks like this

class SignPDFController extends Controller
{
    public function getPDF()
    {
        $client = new DigiSignerClient('client_key');
        $request = new SignatureRequest;
        $request->setEmbedded(true);
        $request->setSendEmails(false);

        $template = Document::withID('document_id');
        $template->setTitle('Site Title');
        $request->addDocument($template);

        $signer = new Signer('[email protected]');
        $signer->setRole('Signer 1');
        $template->addSigner($signer);

        $initials = new ExistingField('key');
        $initials->setContent('VS');
        $signer->addExistingField($initials);

        $response = $client->sendSignatureRequest($request);

        foreach ($response->getDocuments() as $document) {
            foreach ($document->getSigners() as $signer) {
                $signDocumentUrl = $signer->getSignDocumentUrl();
            }
        }
    }
}

The DigiSignerClient and the SignatureRequest classes seem to load fine, but the SignatureRequest needs to load the ExportObject class to extend it.

namespace App\DigiSigner;

use App\DigiSigner\libs\ExportObject;

class SignatureRequest extends ExportObject {

I end up with an error like the following.

Error Class 'App\DigiSigner\libs\ExportObject' not found

Namespaces and use are a little fuzzy for me. If someone can point me in the right direction, I would be delighted.