Up link between 2 schneider's PLC (serial port RS-232 modbus) via WR21 cellular router is a solid solution

I got 2 schneider’s PLC (Quantum and Momentum). I want to communicate between both by using two WR21 cellular router (one per PLC). Schneider’s PLC talk Modbus protocol and the serial connector is RS-232 for each PLC.

Is there somebody who did a installation like this ?
How each router call the other one and automaticly reconnect if connection lost ?
Is the WR21 cellular router really communicate by serial Modbus or it is only Modbus TCP for ethernet application ?