Does your router act as a DHCP server and if so what IP is it giving to your DNS-315?
Open your browser and enter http:// <the IP address of the DNS-315> what do you get?
If you are using windows download a copy of Netscan and use that to see if you can find the DNS-315.
If you can't see the web gui have you tried using another browser?
Does a firewall on your computer stop you accessing the DNS-315?
The above are some things for you to look at. I can't be more specific because you don't give enough details - your question is very similar to 'how long is a piece of string?'
