Hello,
To read Modbus information from a serial port you need to install a package called Modbus Serial Master, since it is not available by default on RUT955. You can do that in two ways:
Way one:
Go to WebUI Services –> Package manager, press Refresh button and then search for Modbus serial master, click the + sign and will be installed. More information can be found here: https://wiki.teltonika-networks.com/view/RUT955_Package_Manager#Package_Manager
Way two: you can manually download the package to your pc and upload it via Package manager -> Upload. You can download the packages .zip file from https://wiki.teltonika-networks.com/view/RUT955_Package_Downloads#Latest_Packages_.28from_07.00.29 and in the zip file look for MODBUS_Serial_master.tar.gz.
After the package is installed, you can find it in Services -> Modbus -> Modbus serial master. Select which serial interface you will be using, enter the serial port options of your device, click save & apply. More info can be found here: https://wiki.teltonika-networks.com/view/RUT955_Modbus#Modbus_Serial_Master
Then you can add a device to your device list, by pressing add and then on the pencil to enter the setting for you modbus device, slave ID and the period of requests to the device. More info here: https://wiki.teltonika-networks.com/view/RUT955_Modbus#RS232_Slave_Devices_List
In the same window you can configure what request will be sent to the device on each period, here you can select the data type, what function will be executed, start register number and how many registers are supposed to be read, you can find more detailed information here: https://wiki.teltonika-networks.com/view/RUT955_Modbus#Requests_Configuration_2
Please note, to get the data from xlink 500 device it should also support Modbus protocol and you should know to which register router should send request and what is the number of registers.
You can also find configuration example of obtaining data via Modbus in the link bellow:
https://wiki.teltonika-networks.com/view/How_to_simulate_Modbus_serial_communication
You can also test your request by selecting the name of the request and pressing the test button.
If you do net get desired results, you can try to add +1 to the register number, since the is no agreement for the starting register number of Modbus devices.
Regards,
Paulius