DHCP - DNS problem

Hi all,
I have a problem with DHCP settings. My gateway worked 
fine with static IP in client. Now I have configured a 
DHCP server on gateway; it seems work normally, the client 
can obtain an IP address, the right IP of default gateway 
and the right DNS addresses. But it can't go on login 
page. Instead, if I write http://<gateway-address>:5280 on 
client's browser, it works.
Any idea?
Thanks
Monica