Hello,
Do you have a Keep persistent connection option enabled in Services -> MODBUS -> MODBUS TCP slave page?
Does your application have an option to create new connection after every Modbus request?
MODBUS in 7.03.4 would simply return 0 value, when RSSI is not available, such as in your case, when mobile antenna is disconnected.
7.04 returns exception, in your case code 4, which indicates, that requested functionality is supported, but for some reason (disconnected antenna) requested data was not returned.
Also, how does your application handle exception messages?
Best regards,