Why can’t I receive my iPhone security codes anymore?

Ok, thanks for the helpful amplification.

Restated, your issue is: “Non-Delivery of SMSs sent by Remote Computers.”

On your phone, verify that Settings -> Messages -> Filter Unknown Senders is “OFF.”

While on this same Settings page, also check for any “Blocked Contacts” which might possibly be culprits.

Beyond that, SMS is a carrier application so non-delivery would need to be addressed by them.

Ensure that they understand that these are computer-generated SMSs which previously WERE being delivered.

It MIGHT be that they are somehow being filtered by an overly-aggressive implementation of you carrier’s SMS SPAM filtering system.

Security codes for WHAT … from WHOM … and sent by what METHOD?

You’re most welcome.

Hope you can get it resolved fairly easily.