The PT104 is a relatively simple device in terms of the communication between itself and the driver on your PC. It does not need a gateway or netmask, only IP address and port, as it communicates directly with an endpoint. Please try specifying the port with the IP address when using the OpenUnitViaIp command, xxx.xxx.xxx.xxx:yyyy, as this may help get the initial communications going over a complex network.
Once communication is established, and the channels set up, the PT104 will transmit data back in a UDP packet for each conversion of data on the channels. Additionally every 3 seconds the driver will check that it still has communication with the device by sending a keep alive packet and expecting a positive response.
If you are still having difficulties the try running wireshark, or similar, on the PC with the driver and look at the packets that are sent out. Additionally run wireshark on a PC connected to a hub, not switch, that is also connected to the PT104 as this will show what packets the PT104 receives and transmits.