On the contrary to the serial port on the CPU module, which is handled by the Linux OS (and is therefore directly accessible from REXYGEN), the 750-652 serial module is not recognized by the Linux OS at all. That's why you do not see it from the command line interface (there is no /dev/tty* device as you mentioned).

The 750-652 module is connected via KBus (the internal bus of Wago I/O) and is not supported in REXYGEN at all. The only possibility is to manually push data to and from the Process Image of the module, as described in Chapter 4 of the 750-652 module documentation. Find it at https://www.wago.com/global/i-o-systems/rs-232-485-serial-interface/p/750-652

I hate to say it, but using any Modbus TCP/RS485 transparent gateway will save you a lot of work. Alternatively, you can even use Wago PFC100 (750-8102) for such a purpose.

Hope this helps at least a bit.

Kind regards,
Jaroslav