Modem doesn't respond to any AT command

  • We have TER-GX 110S and it seems that after a while it doesn't answer to any AT command.

    We have create a software for a customer and there are using it since 8 years.
    But it seems that with some batches of modems (we have different versions, the GX 110S is the one that have currently in the lab of our customer),
    the modem seems to not answer to any AT command.

    our commands are :

    ATV1
    ATE0
    AT&K0
    AT+CPIN?
    AT+CPIN=…. if necessary
    AT+CREG?... wait if still no network
    AT+CGATT=1
    AT+CGDCONT=1,"IP","gprs.swisscom.ch"
    AT#USERID=""
    AT#GPRS=1

    AT#SKTD=0,999,"www.myserver.com"

    if connection is open then start transfer, close the connection, wait some time, restart the process

    but now when we switch off and on the installation (computer + modem), then nothing works. The modem doesn't answer.
    We need to switch off only the modem, and switch it back to get it back.

    How can we solve this problem ?

    15 millions of measure have been send with a lot of different version of modems,
    but now our customers seriously envisage to change the modems as we have trouble with these modems.
    Support will be greatly appreciated.