It is not completely obvious how to turn disable the router functionality on the D-Link DI-624 so that it only operates as a wireless access point. Here is the recipe, which I initially found on dslreports.

Connect to a LAN port (not the WAN port).

Change the 'LAN IP address' to an unused IP address on your LAN.

On Home | DHCP: 
   Disable the DHCP server.

On Home | WAN:
   Set the WAN port to Static IP and then 
   configure the Static IP address to bogus values:
   IP address = 99.99.99.99
   Netmask    = 255.255.255.0
   Gateway    = 99.99.99.100
   DNS        = 99.99.99.100

Reboot the DI-624.