GitHub

This project uses an Arduino Mega to read a Modbus RTU feed through RS485. This example is based on a Huawei / CHINT DTSU666-H.

From my experience it is absolutely needed to have an Arduino Mega or equivalent (needs to have at least 2 serial ports), you won't be able to read the Modbus if Serial is also used by USB Serial ...
GitHub

Modbus RTU ⇒ Modbus TCP/UDP Gateway

Modbus RTU ⇒ Modbus TCP/UDP Gateway Arduino-based Modbus RTU to Modbus TCP/UDP gateway with web interface. Allows you to connect Modbus RTU slaves (such as sensors, energy meters, HVAC devices) to ...