Could your PLC be writing data out of its serial port? If that is the case it will disrupt online monitoring attempt.
You can pause the PLC using DIP switch #4 and then try to go online monitoring. Note that if you re-boot the PLC with DIP Switch #4 ON then the COMM1 on M-series PLC become 9600,8,1,n and you need to modify the setting on TLServer.