How do I turn 2fa off on my iPhone?

See Two-factor authentication for Apple ID:

If you already use two-factor authentication, you can no longer turn it off. Certain features in the latest versions of iOS and macOS require this extra level of security, which is designed to protect your information. If you recently updated your account, you can unenroll within two weeks of enrollment. 

I doubt that is what happened. 2FA always sends a code. And you have the responsibility to enable alternate trusted devices and numbers, so you can always receive a code on one or another of them. If you don’t get the code tap on “didn’t get code” and it will let you choose an alternate way of getting the code.

If you have had it for more than 14 days, you can’t turn it off. It provides additional security for you.