Connecting to Siemens S7-300 PLC
The ISO-on-TCP protocol is used for connection to the PLC, see Connecting to 1200 for your reference.
Input correct TSAP number in the WebHMI connection settings:
for CPU with built-in Ethernet port use TSAP number 0200, 0301
for CPU without Ethernet port and CP343-1 Ethernet communication module use TSAP number 0302