There are a number of things to check, but first I want to be clear on one thing: exactly how is everything connected? Is it phone line - D500 - Netgear - pc or is the pc attached to the DLink or what?
Assuming the pc is connected through both routers, you should make sure that only one of them has DHCP switched on. I would suggest the DLink. Give the Netgear a fixed IP address and exclude that address from the range that the DHCP server issues, eg fix the DLink on 192.168.1.1, the Netgear on 192.168.1.2 and have the DHCP server issue the range 192.168.1.3 - 192.168.1.9