|Okay, for one, is the server providing the DHCP addresses? If it isn't, it's just making things for complex than it should be, at least in my opinion. |
Also, make sure that the server has a static IP address. You said that the modem provides the IP address to the server? That also is a bad practice that needs to be corrected, as having all of the clients connecting to a server for DNS that has a dynamic IP can never lead to good things. The one client may not be able to access it because it has an incorrect DNS address for this reason.
And, if you are running an active directory (which I assumed because you said "domain" in the title), it is even more important for the server to have a static address.
You've been helped by a 16 year old.