I finally solved the problem.
I went on the Apple website https://support.apple.com/apple-id and then on "Tell us how we can help" section to get support.
Then I clicked on iCloud, FaceTime and Messages and they made me choose how I wanted to reach the support.
They called me in under 2 minutes.
I explained the problem which was that FaceTime and iMessage were working fine on my iPhone 8, but there was no way I could log in on my iPad Air 2019, nor on my MacBook Air.
On my iPad there was just an error message every time I tried to log in (in the settings pressing the enter key on the keyboard, or directly on the app), on my MacBook the message said I had no reachable account (I had 5 email IDs and a phone number as reachable addresses).
After sharing my iPad screen with the Apple Support they made me do all those standard and useless procedures I tried before like restarting my iPad and check if I had installed the latest version of iPad OS, which I had.
Obviously nothing worked.
Then the operator told me he would try something different, I had to confirm my identity using my iPhone and he put my call on waiting. I had to wait for like 5-10 minutes, then he told me to try again and it magically worked.
He said something like "your account was blocked because you made too many log in tries" and that they can unblock it just one time (which they just did).
After that I tried to log in on my MacBook Air as well and it perfectly worked.
Instead of spending so much time trying to solve it on my own changing date&time, location... I should have just called the support because there was no way I could solve it myself, they had to unblock it manually.
I hope this helps.