I don't have this option... either on modbus serial or TCP. I'm using codesys 3.5.11.0
As see, in codesys the holding registers are only for writing and the input register just for reading. I don't know for where they take this rule, onde holding register can be read or write in modbus... I have another problem tha I think will be usefull to share here. My master was reading only zeros from codesys. No erros, but the registers were always at zero... So I changed the option "aways uptade variables" (see the attachment) to "enabled". Worked for me.