I don’t know your router D-Link DIR100 have 2 WAN port or not, if its have 2 WAN port try clustering. Otherwise, install an ISA server with 3 network cards, use 2 cards for input line from ISP and the rest is going to be output to the switch, LAN.
There are two clustering technologies.
The first, called the Cluster service, is used to link individual servers so they can perform common tasks. If one server stops functioning, its workload is transferred to the other server.
The second clustering technology, called Network Load Balancing (NLB), is used to make sure a server is always available to handle requests.