Basically, you will pull the data by sending Modbus TCP/IP queries to your RUT955. The RUT955 will then be able to query your controller over serial according to the Modbus slave ID (defined in the TCP/IP query packet, used to identify a slave device). A Modbus TCP Master application is required to query your controllers. I have used QModMaster application on my PC for this purpose. You will have to configure the application to read the specific registers. If configured as I have described above, then connecting to a one port will pull the data from one controller, while connecting to another port will pull the data from the other controller. RUT955 will receive a response from the RTU slaves (controllers) and forward it back over TCP/IP to your server. You will need to reach the RUT955 (For example, via a public IP address) if you wish to read registers remotely. Alternatively, you can query the data from a server on RUT955 local network.
You will need to enable and configure USB to Serial settings (same applies for RS232) in order to establish a communication channel. USB to Serial and RS232 are two separate instances and can be configured via WebUI. The links are at the bottom of my previous response.
Regarding the USB hub, I cannot guarantee that it will work. There can be different issues as you do not know which chipsets are used, so I would not recommend this option. There are also USB to RS232 hubs on the market, but I have not tested them.