Luckily I can still get in on my phone, but it's suddenly using an old, discontinued, company email. I went through all the steps with Facebook and they have "mailed" me a 6 digit code, but I don't know where to use the "unique code". All it tells me on the letter is to go to facebook.com/id
to see the deadline for completing my identity confirmation.
I'm not sure I understand this correctly, it seems like you're stuck in an identity checkpoint for confirming your identity for running ads (I never saw one).
So I assume you have to clear the identity checkpoint by verifying your identity with Facebook. If you don't have access to that company email, you will need to use a next contact or add one via your Facebook settings (you said you have access via your phone) Facebook should in theory be sending that code to any contact point (phone/email) you had setup which you will then use when to sign in.
The six digit code you are referencing is possibly either a 2FA (two factor authentication) code to log you in after entering your password or a OTP (one time password), I'm not sure with the current content from the question.
However, all of this wouldn't matter because you will get redirected to the identity checkpoint.