I believe he said he flashed the stock image before flashing Cortex. I wasn't aware the sim not being recognized was an ongoing issue with Cortex. I was under the impression it had been fixed. I didn't have to do any of those things but maybe it is a problem with some SIMs but not others.
---------- Post added at 11:52 PM ---------- Previous post was at 11:50 PM ----------
One more thing--to be safe I would verify that you are running the specific bootloader and radio builds mentioned in the OP--especially the bootloader.I'm pretty sure there is an issue with my TMO nexus 6p and the OCT 15th version of this ROM. Did a full wipe to Stock October Security Release (proper bootloader, vendor, and radio), flashed twrp, and then ran the install zip. I did this twice and each time, android does not detect a sim card. However, on stock bootup, the sim card is just fine (which is needed to restore backup from cloud since i have two factor authentication enabled).