It sounds a bit like a wrong network setup. Only DHCP doesn't serve any names for servers like your Windows servers, it only provides IP addresses to the Windows servers, nothing else. So if you want to connect to DNS names from the client, you have to setup a DNS for your network as well, which shouldn't be on the client of course. No isc-dhcp or anything else needed on the client, unless you want to make it more complicated then needed. The command
$ ping windowsservername
should work on all clients before calling freerdp. If it's not working, either the name server on your client is not correct (entry in /etc/resolv.conf) or you don't have one at all. If you want to avoid special DNS, you can hardcode the server names into /etc/hosts with their specific IP addresses. I wouldn't recommend having dynamic IPs for any servers either way.