I had the exact same problem. I contacted my ISP because I have a business account and business accounts get priority tech support and even they had no idea what to do about it. I have a westell 7500 modem. This particular modem has a certain feature that lets me configure DNS in certain ways. In the advanced settings of my modem I can access a feature called "DNS Server". Under "DNS Server" I can create a table of domain names and the IP addresses I want them to resolve to. One of the domains in my ISPConfig system is "zandooy.com" so I added zandooy.com to the DNS Server table in my modem with the Local IP address of my Server on my network (ie. zandooy.com /// 192.168.1.21). Then I can just put http://zandooy.com/
in my browser of any other PC on my local network and it gets redirected to my server machine within the same network. BINGO! I can now browse the websites on my server inside my LAN using the domain names. You might try looking for this feature in your router or modem. It may not be labeled the same way if it has it at all.