Facebook and Messenger not loading on iPhone 16

Facebook won't open on my new iPhone 16, as Meta sent me an email to say that someone tried to log into my account on an unknown device, and proudly declared that they had blocked it.

Now having challenges with it on my iPad (can't access it at all) and MacBook Pro as well.

I've tried setting a new password, which either isn't accepted or sends a code to my old iMac and/or stolen iPad. Once it said it sent it to a device with messenger on it.

But Messenger won't work if Facebook isn't logged in!?! On another occasion, I was asked to enter the two factor authentication I had previously set up. All good. Except I hadn't set it up...

Any tips?