The way to configure RUT955 would be by selecting serial mode Over IP and configuring RUT955 in server mode.
Navigate to Services -> Serial utilities -> Over IP. Check the example configuration below:
Next, in order for the device to establish connectivity over WAN, a port needs to be open in RUT955 firewall Network -> Firewall -> Traffic rules, in my example 46798.
To simulate connection I used Hercules utility, one instance being a serial device, the other TCP client.