Today I was struggling with the same problem. in search for a solution i found this thread. Then i tried some things. It doesn't seem to be just a libraries' fault. In addition to 3.5.19, we also have a 3.5.18 version installed, the library versions of Modbus_TCP_Master and Modbus_TCP_Slave are the same, but more than 10 slave channels can be added in 3.5.18