Phpunit: Stubbing a method inside a class in a Functional test

I am performing functional tests with PHPUnit in a Symfony 4.3 project. All my test classes extend from TestCase.

I am having problems to stub the result of a method that makes a call to an external service. I don't want to check that this service works in my project functional tests, so I do the following:

public function testPutEndpoint()
{
    $stub = $this->createMock(ExternalRepository::class);
    $stub->method('put')->willReturn(['data'=> ['data']]);

    {
        list($responseCode, $content) = $this->makeCurl(
           //Here a a curl to the endpoint of my project is done
        );

        $this->assertEquals(200, $responseCode);
    }

Here I see how the code continues going throw the real method ignoring the sub.

So my question is, how can I stub a method that is inside the logic to test, but I cannot call it directly in the test class?