Would it be possible for you to share the message that you get when you try to read Modbus data via ‘test’ button?
There is no need to enter full registers. It should be enough to set the correct function (for example, read holding register), and enter the register number that you you want to obtain. For example, register number 2 (which is Register Address 1). The RutOS uses register numbers instead of addresses. The difference is that addresses start at 0, while numbers start at 1. Basically, if your device uses registers, simply add 1 to get a register number and use it in the modbus client configurations.