You may have to disable the disable the DHCP server on the DIR-655 and change the IP address of the router to match your cisco network. But the DIR-655 is a good router and disabling the DHCP will kill most of the functions ( a wireless switch)
Log into 192.168.0.1 then go to the setup tab and then on the left hand side choose Network Settings to disable the DHCP server and possibly changed the Lan IP address as well. You may also have to Open ports to allow for VPN passs thru.