Making a 56k connection using a nec ncr single subcriber unit

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I have old wheel named made nec named ncr single subcriner unit but can not connect to internet using my internal 56k modem.The response is always the remote computer did not respond.The only way to connect is to use the modem found the unit.Can tell how to connect using by internal 56k,because the provided by the unit can only connect at 19.2k Thank you your help appreciated