How to make I2C work on RS485?
I2C and RS485 are such vastly different protocols with such vastly different signalling needs that they cannot share the same cables.

Further, I2C is not designed for connecting with long cables - it's meant for use between chips on a PCB.

The simplest "Arduino" solution would be to create a new RS485 device on your RS485 bus using another MAX485 and a small Arduino (micro, nano, etc) that responds to MODBUS RTU instructions and controls the relay board for you.