Hello @Customer4376,
with Free plan most customers put RestAPI app on one Website and Angular FrontEnd app on another Website. You can have multiple Free Websites in one account.
With 1x Premium subscription you have included only 1x Premium Website. So if you want 2x Premium Websites you need to buy 2x Premium subscriptions. However, If you prefer to have RestAPI in /API subdirectory, we can create /API directory in your Website, so you can keep RestAPI there like you're used to.
Write us Support ticket in Control Panel and we can test it on Free Website you already use.