Is your problem that OST isn't saving the "Helpdesk URL:" on the settings page or that when you do change it to the "new one" it the new address doesn't answer?
If your problem is the first one:
You can update it via mysql command line using the following mysql query.
UPDATE ost_config SET helpdesk_url='http://new.address.tld' WHERE id='1';
You may have to alter that to fit your install (like if you used a different table pre_fix).
If your problem is the second one:
Since you're running Ubuntu I will presume that means your webserver is probably Apache2. Have you already setup the virtual host in apache or hostname in dns?
