HTTP vs HTTPS for google font error

HI, An SEO company is reviewing my site, and are coming up with an SLL error for my google font which looks like this (sample page but I think it’s the whole site):

Hard Failure A file with an insecure url of “http://fonts.googleapis.com/css?family=Oswald:400,700,300” was loaded on line: 189 of https://www.andesworldtravel.com/our-guides/.
This URL will need to be updated to use a secure URL for your padlock to return.

I am using google font manager, but also had this code in my child theme functions.php:

function add_oswald_script(){
?>
<link href="http://fonts.googleapis.com/css?family=Oswald:400,500,600" rel="stylesheet">
<?php
}
add_action('wp_head', 'add_oswald_script');

I updated that url to be “https://fonts.googleapis.com/css?family=Oswald:400,500,600” but am still getting the error. I also get it if I comment out that script. Can you help me with this? Should I still be using the Google Font Manager plugin or just this script or is there another way of getting that font?

thanks for your help
Nancy