Cannot find the ESN for -C1 product

The -C1 products use a MEID not an ESN.

The MEID (mobile equipment identifier) is a 14 character hex number, the ESN (electronic serial number) is an 8 character hex number.

The MEID is printed on the product's serial number label. You can also read the MEID by issuing the command AT+CGSN. This command returns the MEID and a pseudo-ESN, make sure you provide the MEID to the carrier.