Yes, you need to start TLServer and connect to the PLC via the serial port (RS232 or RS485). Then you will be able to use i-TRiLOGI to connect to the PLC via TLServer and retrieve or reset the Ethernet settings.
TLServer can be started from the Start -> i-TRiLOGI 6 -> TLserver Version 3.1. Once it is started you need to click on the "Serial Port Setup" and enter the test string "IR*" and if all goes well you should receive the "IRxx*" response string.
Note that if your PLC program has changed the serial port to Modbus only, then you may not get a response on the serial port from TLServer. If that is the case, power cycle the PLC with DIP switch #4 set to ON so that the PLC powers up in PAUSE mode and assume the default settings of 38400 bps, 8,1,n. You can then use the TLServer to communicate with the PLC's serial port.