1) Log to your router and browse into Services->Modbus->Modbus TCP Slave
Enable your TCP Slave and assign your device ID. Click save and continue.
2) Go to MODBUS TCP Master and add your slave device.
Your ID should be the same as you assign to your slave.
Make sure that the IP address you assign is the one that is configured for your router( you can check your router IP in Network->LAN)
Click save and continue.
3) Go to MODBUS Data to Server and configure your broker
Click edit and choose MQTT as your protocol.
In your JSON format, you can choose how the data will be presented under your subscriber.
"%a" will represent your register data, which is your analog value in your case.
* you can change the "key" value name if you wish.
Please make sure you have the right broker URL and you publish the data to the right topic