if you have the correct cable connected between the router and PLC you should be able to just send the string as
sendsms phonenumber "message"
the phone number might have to be the intenational number with out the leading "0"'s
to test from the web interface you can use the same command in the "execute command" section
to check what is being passed over the serial interface you can use the analyser if you enable it it should be already configured for asy0 and you should be able to see an the trace section what is being passed to the router