Accessing sub-domain's isn't possible via IP address as easily as just adding the subdomain name... It requires some server-side configuration and this is not something I have ever done in a webserver but I know it can be done, it depends on how much access to the server config you have.
You haven't fully explained clearly how you can access a webserver by IP address but not by its domain name, is the domain name parked and you access the server by IP address?
I have never encountered Plesk in any of my hosting, only ever had cPanel so I can't help you with any possible settings needed in the interface.
You can find plenty of information about configuration of the server settings but that is something that requires system access to certain folders that wouldn't normally be accessible to external users, its something a sysadmin would need to add or mount and does require configuration.
What we do not know, as pointed out, the exact details of how your domain name and IP address work together, is it as previously asked, a parked domain or did you buy your hosting and domain from the web host or did you buy them separate?