Missing component on the Usb to RS485 converter
Looks like it is missing a resistor with the marking 121 on it (120Ω)

There must be one at each end of the buss. So if this is just point to point, then it may be added externally to the screw terminals. (kudos to GK for the nudge)

I would use a logic probe, scope or DMM on the MAX chip pin 1,2,6,7 and check status. If RE is low then RO = voltage(Pin 7-pin 6) "1"=+ve, "0"=-ve