public class RestClient {
    private static final String BASE_URL = "твой юрл";

    private static final String username = "sportExtern";
    private static final String password = "xsgrnt4dNcpny";

    private final ApiInterface service;

    public RestClient() {
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(BASE_URL)
                .client(new OkHttpClient())
                .addConverterFactory(SimpleXmlConverterFactory.create())
                .build();
        service = retrofit.create(ApiInterface.class);
    }

    private void getData(String user, String password, Callback<GamesData> callback) {
        Call<GamesData> call = service.getData(user, password);
        call.enqueue(callback);
    }

    public void getGamesData(Callback<GamesData> callback) {
        getData(username, password, callback);
    }
}
репозиторий
public class GameRepository {
    private RestClient restClient;
    private List<Branch> branches = new ArrayList<>();

    private MutableLiveData<List<Branch>> branchesLiveData = new MutableLiveData<>();
    private MutableLiveData<List<Game>> gamesLiveData = new MutableLiveData<>();

    public GameRepository(RestClient restClient) {
        this.restClient = restClient;
    }

    public LiveData<List<Game>> getGamesLiveData() {
        if (gamesLiveData.getValue() == null) {
            loadGamesData();
        }
        return gamesLiveData;
    }

    private void loadGamesData() {
        restClient.getGamesData(new Callback<GamesData>() {
            @Override
            public void onResponse(@NonNull Call<GamesData> call, @NonNull Response<GamesData> response) {
                if (response.isSuccessful()) {
                    GamesData gamesData = response.body();
                    if (gamesData != null) {
                        branches.addAll(gamesData.getBranchesList());
                        branchesLiveData.postValue(branches);

                        List<Game> games = new ArrayList<>();
                        for (Branch branch : branches) {
                            games.addAll(branch.getGames());
                        }
                        gamesLiveData.postValue(games);
                    }
                } else {
                    L.d("responseCode " + response.code() + "response.isSuccessful " + response.isSuccessful());
                }
            }

            @Override
            public void onFailure(@NonNull Call<GamesData> call, @NonNull Throwable t) {
                L.d("Error " + t);
            }
        });
    }
}

и в фрагменте подписываешся на ливдата и отдаеш в адаптер
public class ScheduledGamesFragment extends Fragment implements GamesAdapter.GameClickListener {
    @BindView(R.id.fsg_game_list)
    RecyclerView gamesList;
    @BindView(R.id.fsg_dates_list)
    RecyclerView datesList;

    private GamesAdapter gamesAdapter;
    private DatesAdapter datesAdapter;

    private Unbinder unbinder;

    public static ScheduledGamesFragment newInstance() {
        return new ScheduledGamesFragment();
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_scheduled_games, container, false);
        unbinder = ButterKnife.bind(this, view);

        getActivity().setTitle(getString(R.string.sgf_scheduled_games));

        initGamesList();
        gamesAdapter = new GamesAdapter();
        gamesAdapter.setGamesClickListener(this);
        LinearLayoutManager managerGames = new LinearLayoutManager(getActivity());
        gamesList.setLayoutManager(managerGames);
        gamesList.setAdapter(gamesAdapter);

        datesAdapter = new DatesAdapter(getListDates());
        LinearLayoutManager managerDates = new LinearLayoutManager(getActivity());
        datesList.setLayoutManager(managerDates);
        datesList.setAdapter(datesAdapter);

        return view;
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        unbinder.unbind();
    }

    @OnClick(R.id.fsg_btn_choose_league)
    public void onClickChooseLeague() {
        startActivity(new Intent(getActivity(), AddTeamActivity.class));
    }

    @Override
    public void onGameClick(int position) {
        startActivity(new Intent(getActivity(), GamesActivity.class));
    }

    private void initGamesList() {
        LiveData<List<Game>> gamesLiveData = App.getInstance().getGameRepository().getGamesLiveData();

        gamesLiveData.observe(this, new Observer<List<Game>>() {
            @Override
            public void onChanged(@Nullable List<Game> games) {
                if (games != null) {
                    gamesAdapter.setGames(games);
                }
            }
        });
    }
}