Have to remember that any port forwarding on the 655 in AP mode will not work or should be used. Any features of the 655 router is effectively disabled in this mode.
I would recommend that you bridge the ISP mode and use the 655 as a full on router so you can take better advantage of it's features and it will probably clear your problems up. This has the ability to handle and manage more things than your ISP modem seems to be able to do. If your ISP modem can't be bridged, try using the DMZ on the modem to input the IP address the 655 gets from the modem after you put the 655 back into Router mode.