What are the possible reasons for the failure of PPP dial up?

  1. Does the serial port be occupied by other serial communication tools when dialing?
  2. Are the baud rate of module, modem and dial up the same?
  3. Is the APN configuration right or wrong in the modem?
  4. Is the device five-wire designed or three-wire designed? Does the device open or close the hardware flow control?
  5. Is the tested SIM card being PIN locked? Is SIM card in arrearage?
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Is there any difference between module and modem?

As for example I am using EC25 mini PCIe, should I call it a module or modem?