How to use AT+CFTRANTX command in SIM7100
When having trouble in getting started with a modem, it's usually best to connect to the modem in "interactive mode", to verify that you're sending the right commands and getting the right responses.

Have you made sure that your modem/module's UART is at the right logic level to interface directly with your Pro Mini? How about the supply voltage? (Post a link to the product). If you're done with all that, then connect only the modem to the Pro Mini:

5V/3.3V -&gt; Vcc (depending on the module's supply voltage)
D6 -&gt; Modem TX
D7 -&gt; Modem RX
Gnd -&gt; Gnd


Then, upload the following to your Pro Mini:

#include &lt;SoftwareSerial.h&gt;

SoftwareSerial fserial(6, 7);

void setup(void)
{
    Serial.begin(57600);
    // I assume this is the default for your modem
    fserial.begin(115200);
    Serial.println("Starting.");
}

void loop(void)
{
    while (fserial.available())
        Serial.write(fserial.read());
    while (Serial.available())
        fserial.write(Serial.read());
}


This should basically transfer everything you send from your Serial Monitor to the modem, and in the reverse, transfer everything the modem sends to your Serial Monitor.

After uploading the sketch, in the Serial Monitor, set the Line Ending at the bottom to Carriage Return. Then you can first send the simple AT and should receive OK from the modem, to confirm that your setup works. Then to test your file reading code, send your command AT+CFTRANTX="c:/data.txt" and see what you get.

If you can confirm this works, writing code to do this automatically should then be much clearer. Or at the very least, you'll have ruled out a whole bunch of possible problems.

(Note: the Arduino may not work reliably with 115200 baud on Software Serial. So you may want to change the modem's baud rate quickly with AT+IPREX=9600. And then modify your sketch accordingly and re-upload.)