ok, let’s start first with the information you will give us
and that is the name of the domain that you bought (to check if everything is OK with NS).
But in short, if everything is ok with NS (they refer to this hosting)
then
read Option 2 here
and select Addon Domain
then
upload files with FTP client or file manager from Client Area (monstaFTP)
as you have done so far for your free subdomain
but this time you will upload the files in htdocs from your domain (it starts with the name of your domain <your domain>/htdocs)