DFRobot SIM7000C problem of setting APN

Hello,i am a dfrobot sim7000c user from Taiwan

I want to setting the APN of the board. FarEasTone telecom is the supplier of the nbiot simcard that i used.
And its nbiot apn is "nbiot".

I used ATTest sample code to execute my command.After entering the loop function,i type some AT Commands and the result is:

AT+CGDCONT=1,"IP","nbiot" ,
OK
AT+CGNAPN
+CGNAPN: 0,""

OK

it showed that i did not set the APN successfully,therefore,i tried another command.

AT+CGDCONT=1,"IP",""
OK
AT+CGNAPN
+CGNAPN: 0,""

OK
I obtained same result.
but in the AT+CGDCONT=1,"IP","nbiot" command,the board response "OK" to me.
It seem like i successfully set it,so I would like to ask where my problem is and how to solve it.
thank you.