Hello,
It requests additional customization in files, which is not included in our basic support.
Do the next via FTP:
add $with_buy_now = apply_filters('etheme_sticky_add_to_cart_buy_now_hide_mobile', $with_buy_now);
into wp-content/plugins/et-core-plugin/app/models/customizer/functions.php – http://prntscr.com/1rhpcjf
and this add_filter('etheme_sticky_add_to_cart_buy_now_hide_mobile', '__return_false');
into xstore-child/functions.php
Regards