It sounds like your sm5100b is configured for 115200 baud which SoftwareSerial doesnt handle. I had the same issue. To resolve you need to load a blank sketch onto your arduino (define empty setup() and loop() functions). And run a jumper from pin 2-3 and 1-4. then use a terminal application (hyper terminal on win/ screen on mac) and connect to the usb serial device at 115200 baud.
Then send the following command:
AT+IPR=9600
and restart, then you can talk to it at 9600 baud