Arduino UNO and RS485 Communication Problem
I would suggest you simplify your setup to the point where you have a 'working baseline  configuration', and then start building the system up one step at a time from there.

Start by eliminating the SoftwareSerial interface as a possible contributor to the problem.  Get an Arduino Mega with multiple hardware serial ports. 

Next, verify that you can send characters to the selected hardware serial port and receive those same characters back when the Txn and Rxn lines (the 'n' here is the selected hardware serial port number) are connected via a jumper wire. This is called a 'loopback' configuration and is a basic serial comms troubleshooting technique (software serial ports don't support simultaneous transmit and receive, so they don't support loopback testing).  Don't proceed until you have this step completed successfully.

Next, get a second RS-485 module and extend your previous 'loopback' configuration with the hardware serial port on the Mega to include the two RS-485 modules.  Then you should be able to send characters out to one RS-485 module and receive the same characters back from the other, by connecting TXn to DI on one of the two RS-485 modules, and Rxn to DO on the other.

At this point you will have verified all the communications connections, so you can be confident that whatever you send will actually show up at the DO port of the RS-485, and whatever shows up at the DI port of the RS-485 will actually make it back to your serial console.  Now you can connect the sensor and figure out any issues do to data formatting, command sequences, and the like.

This basic 'cut the problem in half' building block technique will work on any system, large or small, simple or complex.

Frank