I expected to be able to change the IP to mydomain.com
IP address to domain name is being assigned from upper level - either from any kind of web server control panel or Apache2 virtual host.
PHPMyAdmin is just a web software for management of MySQL database. Do not mess it up without knowing - there is no way to undo if you delete a Table there.
It takes a bit time for IP to domain name propagation to happen. Its a kind of redirection through NameServer. Redirection is not the right term, still.
You can ask your webhost's support to configure it for you. Even it is unmanaged, this part is managed the web hosting companies.