private void APIRegisterDisplay() {

    RegisterDisplay registerDisplay = new RegisterDisplay(abc, xyz ...);
    
    apiService = RetrofitClient.getClient(cmsAddress).create(APIService.class);
    apiService.Register(registerDisplay).enqueue(new Callback() {
        @Override
        public void onResponse(@NonNull Call call, @NonNull Response response) {
            if (response.isSuccessful()) {
                assert response.body() != null;
                JsonObject json = response.body().getAsJsonObject("response").getAsJsonObject("display").getAsJsonObject("$");

                Gson gson = new GsonBuilder().setPrettyPrinting().create();
                String string = gson.toJson(json);
                Display display = gson.fromJson(string, Display.class);

                if (display != null) {
                    if (display.getCode().equals("0")) {
                        //code == 0 Display is active and ready to start
                        Toast.makeText(getApplicationContext(), display.getMessage(), Toast.LENGTH_SHORT).show();
                        Log.d(TAG, "get display successful");

                        checkConnect = true;
                    } else {
                        Toast.makeText(getApplicationContext(), display.getMessage(), Toast.LENGTH_SHORT).show();
                    }
                } else {
                    Log.d(TAG, "failed");
                    Toast.makeText(getApplicationContext(), "Unable to connect to CMS!!", Toast.LENGTH_SHORT).show();
                }
            }
        }

        @Override
        public void onFailure(@NonNull Call call, @NonNull Throwable t) {
            Log.e(TAG, "REGISTER_DISPLAY:" + t.getLocalizedMessage());
        }
    });
}

Các bác chú ý biến checkConnect. Biến này để kiểm tra api trả về display.getCode() có bằng 0 (nghĩa là thành công) hay không.