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:
Is there something else I'm missing?