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.
Thank you for contacting Teltonika networks Crowd support.
From the pdf file shared I can note that you are using an old firmware on RUT955. Kindly update to the latest firmware for this solution is based on RUT9_R_00.07.02.3. To update the firmware, go to
The following links shows step by step on how to configure Modbus and RS232.
For testing purposes you can use a Modbus Simulator of your choice like ModRsim2.
