I have solved my problem using a trick.
I explain the sequence:
1. there are situations where MB_TCP_Server, detects that there is no connection (MB_TCP_Server.xConnected=False) in this case, there is a mechanism that activates, deactivates the block until get the MB_TCP_Server has connected (MB_TCP_Server.xConnected=True).
2. There are situations when MB_TCP_Server, detects error (MB_TCP_Server.xError=True), in this case, there is a mechanism that activates, deactivates the block until getting the correct connection (MB_TCP_Server.xConnected=True and MB_TCP_Server.xError=False), the connection is established after about 60 seconds.
3. There are situations where MB_TCP_Serve does not detect that there is no connection (MB_TCP_Server.xConnected=True, but the Modbus client equipment is not operative), in this situation what I have had to do is to force the PLC to deactivate the block, so that the mechanism of point 1, starts to work, The described in this point implies that I must force the disconnection every certain time (for example every 2 minutes).
In this way I make sure that there is always a connection with the Modbus client.