Getting Connection refused: connect; nested exception is java.net.ConnectException: with MockRestServiceServer

Question

I want to mock RestTemplate object which is calling another microservice and receiving configuration for a particular device:

ResponseEntity<ConfigurationResponse> configurationResponse = restTemplate
        .getForEntity("http://localhost:8081/configuration/serial/" + dto.getSerNum(), ConfigurationResponse.class);

I am mocking above RestTemplate like:

mockServer.expect(requestTo(
        "http://localhost:8081/configuration/serial/" + device.getSerNum()))
        .andExpect(method(HttpMethod.GET))
        .andRespond(withSuccess(toJson(
            ConfigurationResponse.builder()
                .ip("192.168.1.1")
                .netMask("255.255.0.0")
                .build()),
            MediaType.APPLICATION_JSON_UTF8));

but after the test start I am receiving an exception:

org.springframework.web.client.ResourceAccessException: I/O error on GET request for "http://localhost:8081/configuration/serial/XXX-BBB-KKK": Connection refused: connect; nested exception is java.net.ConnectException: Connection refused: connect

I am instantiating RestTemplate and MockRestServiceServer like:

class DeviceServiceTest {

    private DeviceService deviceService;

    private DeviceRepository deviceRepository = mock(DeviceRepository.class);

    private RestTemplate restTemplate = new RestTemplate();

    private MockRestServiceServermockServer = MockRestServiceServer.createServer(restTemplate);

    @BeforeEach
    void setUp() {
        deviceService = new DeviceService(deviceRepository);
        mockServer = MockRestServiceServer.createServer(restTemplate);
        restTemplate = new RestTemplate();
    }
}

I used below example from link below how-mock-rest-request

but it did not bring a desirable effect. I will be grateful for a piece of advice on how to fix my mock to establish a connection in my test.

EDIT

Basing on the topic click I know that I suppose to have the same bean of RestTemplate in service and in the test but honestly I don't know how to make it happened. I am instantiating a RestTemplate object in a method which I am testing.