- Make sure the device is set to RNDIS+UVC or RNDIS (seems you already did that)
- Make sure you have installed the Flir device driver. (Otherwise it doesn't detect the USB device as a RNDIS, but Serial port).
- The IP address can be 192.168.0.2 or 192.168.250.2; I don't know how the cam decides on its address.
- Auto configuration never worked for me, so I had to manually set 192.168.250.1 as the local IP address. Use 255.255.255.0 as the network mask, and don't set a gateway or DNS.
Make sure the cam can be ping'ed. ("ping 192.168.250.2" for example). Only if that works try to connect with an FTP client.
(FYI "Gateway" doesn't matter; the cam does not provide internet. 192.168.x.2 isn't the gateway IP address, but the IP address of the cam.)