The IMEI on the label of SIM800 is not matching IMEI from AT command AT+CSN why is this ? I checked both IMEI in an online checker and they are both registered to SIMCOM
That would only be 2-digits (IMEISV). The 14-digit ID number is different e.g the two IMEIs I get
IMEI.info: 860719029982224
IMEI.info: 863584032959818
Only SIMCOM can clarify how it's possible. Firmware update, at least with regular release should never alter IMEI. A possible explanation could be a counterfeit module.
It would be helpful if you could inform the origin of this module, whether purchased it in the original sealed box from a reputable supplier, or anywhere.