I will be creating a connection between my Linux server and a cellular modem where the modem will act as a server for serial over TCP.
The modem itself is connected to a modbus device (industrial protocol) via an RS232 connection.
I would like to use pymodbus to facilitate talking to the end modbus device. However, I cannot use the TCP modbus option in PyModbus as the end device speaks serial modbus (Modbus RTU). And I cannot use the serial modbus option in Pymodbus as it expects to open an actual local serial port (tty device) on the linux server.
How can I bridge the serial connection such that the pymodbus library will see the connection as a local serial device?
I actually was working on something similar and decided to make my own Serial/TCP bridge. Using virtual serial ports to handle the communication with each of the modems. 
I used the minimalmodbus library although I had to modify it a little in order to handle the virtual serial ports.
I hope you solved your problem and if you didn't I can try to help you out.
Pymodbus (Serial) over a tcp serial connection