If the module responds +CPIN: READY after powering on, but after a while it responds +CPIN: NOT READY, you should consider the following:

1) RF interference

1. Use AT+CFUN=4,1 to disable the RF function and see if the issue persists
2. Put the antenna close to the SIM card and SIM card signal line, then see if the issue occurs more frequently
3. Check if there is any electrical/magnetic field around the testing field, e.g. high voltage transmission lines or high-power wireless devices, and try using a shielding case to cover the SIM card and all the SIM signal lines
4. Try to connect the pins of SIM card signal (SIM_DATA/SIM_CLK/SIM_RST/SIM_VDD) to ground via 15~33PF capacitors to eliminate RF interference

2) Hardware design issue

If the VBAT trace is too close to the SIM card signal line, the module can still recognize the SIM card because of the low ripple of VBAT when powering on, but when the module starts to register with the network, the ripple will become higher and interfere with the SIM card signal line, which may stop the module from recognizing the SIM card. You can cut off the module’s power supply and use an external power source to directly feed to VBAT pin of the module and see if this resolves the issue

3) Poor quality SIM card

1. Change to another SIM card and see if this resolves the issue
2. Put the problematic card on the Quectel EVB or cell phone to see if the same issue occurs