Always "Failed to verify your identity. Try again."

Yes, that is the title. I have tried so many times for 3 days just to log in the game with Apple Account. But the only thing I can do is tapping over and over on continue button and a bunch of notifications come to my phone. It do send 2FA code while the log in screen says that error. I am finding a solution but none of them can fix. Even changing the password, using VPN, cellular data, adding 2FA code at the end of the password cannot make any sense. BUT when I use another device (not iPhone) it is normal.

By the way, I am using iPhone 6 Plus, running iOS 12.5.7. Before that, I could log in normally without any error and even many times.

I think the release of iOS 18 might affect something to Apple login site or my account has some problem? I want to hear something from you all. (and of course say no with changing phone, I am a student and can only change phone next year.)