Sleep mode functionality is only available in RUT850 routers.
If you need to turn off router at certain voltage levels, try to use analog input to measure router input voltage and configure a rule, which will change relay state from ON to OFF. If you wire power line through relay, this will cut the voltage and router will turn OFF. However, this solution only works if you need to turn OFF the router. To turn it ON again, you will need additional circuit, which can hold router ON long enough, until i/o service will be activated and relay will change state to ON.
All information about RUT955 Input/Ouput functionality is located in Teltonika wiki: https://wiki.teltonika.lt/view/RUT955_Input/Output