Magento – Rest API Acess denied 403 for customers and orders

I can give you workable example:

System / Web Services / REST Roles

    Role info: 
Role Name: admin
Password: 123123
    Role API resources:
Resources access: All
    Role Users:
I have assigned admin user

System / Web Services / REST Attributes / Select {Admin}

Resources access: All

System / Web Services / REST OAuth Consumers

My magento url is http://mg1910.local.dev/

My http://mg1910.local.dev/oauth_admin.php is following:

<?php

/* live server */
$host = 'http://mg1910com.local.dev/';
$consumerKey    = '7e14cd85d05456c3e4de9e3c5c5f61e4';
$consumerSecret = 'f8aab713fa50504f0fac99d564ecaf7a';
/* << live server */


$callbackUrl = "http://mg1910com.local.dev/oauth_admin.php";
$temporaryCredentialsRequestUrl = $host . "oauth/initiate?oauth_callback=" . urlencode($callbackUrl);
$adminAuthorizationUrl = $host . "admin/oauth_authorize";
$accessTokenRequestUrl = $host . "oauth/token";
$apiUrl = $host . "api/rest";

session_start();
if (!isset($_GET['oauth_token']) && isset($_SESSION['state']) && $_SESSION['state'] == 1) {
    $_SESSION['state'] = 0;
}
try {
    $authType = ($_SESSION['state'] == 2) ? OAUTH_AUTH_TYPE_AUTHORIZATION : OAUTH_AUTH_TYPE_URI;
    $oauthClient = new OAuth($consumerKey, $consumerSecret, OAUTH_SIG_METHOD_HMACSHA1, $authType);
    $oauthClient->enableDebug();

    if (!isset($_GET['oauth_token']) && !$_SESSION['state']) {
        $requestToken = $oauthClient->getRequestToken($temporaryCredentialsRequestUrl);
        $_SESSION['secret'] = $requestToken['oauth_token_secret'];
        $_SESSION['state'] = 1;
        header('Location: ' . $adminAuthorizationUrl . '?oauth_token=' . $requestToken['oauth_token']);
        exit;
    } else if ($_SESSION['state'] == 1) {
        $oauthClient->setToken($_GET['oauth_token'], $_SESSION['secret']);
        $accessToken = $oauthClient->getAccessToken($accessTokenRequestUrl);
        $_SESSION['state'] = 2;
        $_SESSION['token'] = $accessToken['oauth_token'];
        $_SESSION['secret'] = $accessToken['oauth_token_secret'];
        header('Location: ' . $callbackUrl);
        exit;
    } else {
        $oauthClient->setToken($_SESSION['token'], $_SESSION['secret']);
        $resourceUrl = "$apiUrl/products?limit=3";
        //$oauthClient->fetch($resourceUrl, array(), 'GET', array('Content-Type' => 'application/json'));
        $oauthClient->fetch($resourceUrl, array(), 'GET', array('Content-Type' => 'application/xml', 'Accept' => '*/*'));
        $productsList = json_decode($oauthClient->getLastResponse());
        echo "<pre>";
        print_r($productsList);
    }
} catch (OAuthException $e) {
    print_r($e->getMessage());
    echo "<br/>";
    print_r($e->lastResponse);
}

As you can see my script receive 3 products.

One order can contain multiple products, which are represented as order items in the context of order. With order APIs you cannot get order list with order items in one request. But it can be done as follows:

1. Get list of order IDs which you are interested in (e.g. using request provided in the question)
2. To get product details use \Magento\Sales\Api\OrderItemRepositoryInterface::getList($searchCriteria). Via REST it is available as GET <base_url>/V1/orders/items. Iterate through orders from the previous step and for each one get its items, in search criteria specify filter by order ID