??? You seemed to be very confused about RS232 vs IR.

RS232 is a serial data connection. It has 3 lines (RX, TX, and GND). It works in a similar way a network jack would work.

IR is "Infrared" and is used to transmit data wirelessly thru IR light. The electrical signal for this IR data can be transmitted over wire with 2 conductor. When you see a 3 conductor jack for IR, that is meant for an IR receiver. The 3rd line is power to work the IR receiver.

These two things are "NOT" the same or compatible.

So there is no connection from an MRF-350, that you can connect to a RS232 port. The MRF-350 doesn't have any RS232 ports. It only has IR outputs.

Now about your MRX-2. It can be used in a Complete Control (CCP) and it can also be used in a Total Control (TC) setup. In a CCP setup, it only works IR - the serial port is only available for use in a TC system.

So can you explain how you are using RS232 currently on a CCP system with an MRX-2. When that simply is not possible.
Unless you forgot to tell us you are currently using an old MSC-400 to do RS232 serial with your CCP setup. Unfortunately, that old MSC-400 (and a few old keypads) were the only way to do serial control in CCP. URC doesn't currently have anything that supports RS232 anymore in CCP lineup - as far as I know.