Be advised, that the RUT955 is fully supported by official openwrt. Based on this, Being a freelancer, I developed custom firmware for a large fleet of RUTs to be used as data concentrators in IoT. Lower level devices are connected via ModBus/RTU, no problem. In your case, you should try to connect a ModBus Simulator, i.e. ModRSsim2, to your RUT, to verify the polling, and electrical setup.