Cisco Routers :: 887VA Cannot Expect Client To Make Change Every-time

I have a curious issue on a customer site where the above components are installed.The issue being that the iPhones the users utilise cannot connect to the mail server via WiFi, they are forced to connect with their 3G connection. The core issue seems to be the 887VA, as when I've installed 'lesser' branded ADSL routers, the connection seems to be fine.The iPhones are set to access the mail server via the URL: mail.customer.com. Should I change this to the internal IP of the server 192.168.12.200, the mail connection works fine. However I cannot expect the client to make this change every time they arrive or leave the office. Presently I have them disable WiFi when on premises.