The PLC cannot use the MODEM to directly connect to the Internet.
Howver, if you connect the PLC to a modem, and connect the PC running TLServer to the modem, then the connection between the modems doesnot require any username and password. But that has no effect on the way TRiLOGI client login to TLServer. In this case the modems only replace the serial cable between the PC and the PLC.