Seems like this might be a configuration or syntax configuration with FTP program. Being the the same side with the PC and NAS, the FTP program shouldn't have any issues connecting.
Is uPnP enabled on the 655?
The ISP modem shouldn't impact the LAN side connections. Are you getting a private or public IP address on the WAN side under Status/Device Info on the 655?
One thing to try is to set a static IP address on the PC and NAS, then directly connect the PC to NAS with out the router. See if you can connect then or try a switch. Switches are good things to have.
Also disable any 3rd party Firewall programs temporarily while testing.
Set SPI NAT on the 655 to EndPoint Independent to be sure. Shouldn't have to but just in case.