Hello,
There is no option to move only custom tab under the add to cart button, you can move all the tabs below the share icons http://prntscr.com/qmzoka . To do this go to Royal > Single product > Location of product tabs > Next to the image.
If you want to display only custom tab content and below the add to cart button then submit additional customization request here becasue additional customization is not included in base support.
Regards