Login to Wordpress Admin via IP but not domain name

I can login to wp dashboard using my ip address, but not my domain name, i.e:

12.345.67.89/wp-admin works ok mydomain.com/wp-admin doesn’t work

I just get taken back to the login page again when I enter correct login details. (If I enter incorrect ones I get the usual error message).

I’m assuming it is some settings somewhere, but I don’t know where.

I’ve edit my wp-config file:

define(‘WP_HOME’,‘http://mydomain.com’); define(‘WP_SITEURL’,‘http://mydomain.com’);

Is there something else I’m missing?