A couple of years ago I had to start using two factor authentication because of the high security rules at a customer, then this method stopped...

A couple of years ago I had to start using two factor authentication because of the high security rules at a customer, then this method stopped working when I got new devices (I have a Mac, an iPhone, an iPad) but as long as I kept my old iPad I could log in using a "safety code" (I don't know the exact word, my Facebook is in Swedish), so I used that method, even though I suspected that was wrong.

Half a year ago I got a new iPad and I was smart enough to log in on that device using my old iPad, but then I sold the old iPad, so that method isn't working any longer. To make things worse, Facebook is announcing that they will soon demand two factor authentication for everyone. Today I couldn't log in to the Messenger app on my Mac because of this.

I have tried to follow the available instructions, but I always get stuck. Facebook "says" it send me a code to my iPhone, (Messages), but I get no such code. I have tried to register my husband's iPhone number, but that doesn't work either. I have no clue about how to solve this matter. Do you have any suggestions of how to solve this?

1. Check Mobile Network Connection: –> No problem.
2. Resend Code: After requesting a code, wait for a few minutes to see if it arrives. If it doesn't, you can click on the "Resend Code" option to have Facebook send another authentication code to your registered mobile number. –> No, I have tried to solve this for so long, and I never get a code.
3. Check Spam Folder: Occasionally, SMS messages from Facebook may get filtered into your spam folder. Check your iPhone's spam or junk folder to see if the code has been redirected there. –> I have no spam filters on my iPhone’s Messages.
4. App Permissions: Make sure the Facebook app on your iPhone has the necessary permissions to receive SMS messages. Go to your iPhone settings, find the Facebook app, and ensure it has access to messages and notifications. –> I don’t understand this? Facebook claim they send me SMS to my iPhone, Messages. Not to the Facebook Messenger. And I can’t find any such settings anyway.
5. Verify Phone Number: Double-check that the phone number linked to your Facebook account is correct. You can do this by going to Facebook settings > Security and Login > Two-Factor Authentication. Ensure that the phone number listed is accurate. –> I have double-checked the phone number about 100 times…
6. Try a Different Contact Method: Facebook often provides alternative methods for receiving authentication codes, such as using a code generator app like Google Authenticator or Facebook's own Code Generator feature. You can set up Code Generator in the Facebook app under Settings > Security and Login. –> Whatever I try to do, I seem to be stuck with the two-factor authentication method that I started to use a couple of years ago but is no longer working for me. I can’t change how to receive authentication codes because I’m stuck with having to use the Duo Mobile app for authentication.
7. Contact Facebook Support: If none of the above steps work, you can try reaching out to Facebook's support team for assistance. They may be able to help you troubleshoot the issue further. –> Good Luck with that; how do I even get in contact with them? I tried to connect with them half a year ago, and they just don’t answer.
8. Backup Codes: If you had previously set up backup codes for two-factor authentication, you can use those codes to log in. These codes are usually generated when you initially set up two-factor authentication. –> There’s no place to add a backup cod. I do have some kind of reset code, but it doesn’t work.
9. Recovery Options: Review and update your recovery options on Facebook. This includes adding trusted contacts who can help you regain access to your account in case of issues. –> Hm? That’s more for a future problem, right? I will check into it, but it won’t help me now.

However, I think that maybe the key is to solve this problem via the Duo Mobile app? 

From their homepage:

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Duo Restore

Provide your users with the ability to back up and restore their Duo Mobile app with Duo Restore. This feature allows Android and iOS Duo Mobile users to back up their Duo-protected accounts and recover them when they get a new device — no help desk ticket needed.

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It seems one has to do something special when one gets a new device. It's a pity no one told me that from the beginning…

Oh, I'm sorry for a late reply, I live in Sweden, so the time zones…

No, so far it works on my Mac. But Facebook Messenger stopped working on my Mac yesterday, because of this two factor authentication…

Facebook has announced that soon one will have to use two factor authentication also on computers… so when that happened I don't know what to do. A lot of my work depends on having access to Facebook.

Now I will be gone again. Sorry. It's late over here.

I don't really see how you think uninstalling the app would work? Even so, I uninstalled it, reinstalled it, and of course, the same result: I need to do two factor authentication to log in, which I can't do.

It started with being locked out from Facebook on an iPad. Then what happened is that I got locked out also from the Facebook Messenger app on my Mac computer.

It will eventually be a problem for all my devices and the Mac, since Facebook has declared that they will move the users to two factor authentication all over… When that happens, I will be totally locked out.