1- In functions.php
add_theme_support( 'post-thumbnails' );
add_image_size('my-custom-thumb', 210, 210, true);
2- Note (https://codex.wordpress.org/Function_Reference/set_post_thumbnail_size)
This function will not resize your existing featured images. To regenerate existing images in the new size, use the Regenerate Thumbnails (http://wordpress.org/plugins/regenerate-thumbnails/) plugin.
If that doesn't work, then I would probably triple check if there is an image uploaded for that $id on the mobile version (just check manually from the backend), and if its custom-sized version exists in the directory you set for uploads. You can manually check for the thumbnail in the database too (check this: https://stackoverflow.com/a/28643825/1427624), and then your custom-sized thumbnail should be in the same directory of the url value in the guid column.
If all of this checks out, and it's still not working, but you can successfully get an image with the size parameter "medium" or "thumbnail", then maybe you would use an automatic cropper either on the server side or the frontend side to dynamically crop that image to your custom size (a.k.a. the "out of good solutions" solution).