ISPConfig 3.1 problem

At my office, I have recently had installed a second internet connection (Ultra Fast Optic) and given how fast it is, I decided to setup a server running ISPConfig 3.1 (Ubuntu 16.04) but have a strange problem. I have no problem accessing websites set-up and hosted on this ISPConfig server from external PCs (such as PCs connected to the internet through my older internet connection) but I cannot access these same websites from any desktop or laptop PCs that are on the same local network as the server. The ISPConfig server sits behind a fairly standard router with all incoming traffic on ports 21, 22, 80, 443 and 7878 forwarded to the ISPConfig server on a local IP.