Can't add Gmail to Mail.app, iOS

I tried to add Google Gmail to Mail.app on iOS 11.3, my steps are: Settings -> Accounts & Passwords -> Add Account -> Google, then input my Gmail address(****@gmail.com), my password, 2-Step Verification code. Then there is a screen with 4 switches. On this screen, all 4 switches are faded that I can't enable or disable them like the picture below.

This screen flashes out within 1 second and my account is still not added.

I've tried some other options.

1. I added my account on another iPhone with iOS 11.3, can't be added either, which means it's not my iPhone's problem.

2. I can use Gmail.app/Gmail website/Gmail on other 3rd party mail apps and they work fine, which means my Gmail account has no problem.

3. I can add other emails like Microsoft exchange, only Gmail can not be added.

4. My Google Account/Gmail on my Mac or PC is OK too.

5. I created a new account from Google.com and tried to add it to my iPhone, to my surprise, this new account won't be added either.

6. I've restarted my iPhones for 6 or 7 times, no difference.

6. I restored my iPhone and set it as an new iPhone, my Gmail account still can't be added.

I've done these options on my 2 iPhones: iPhone SE(64GB, A1723) and iPhone 6(16GB, A1586), they all do the same thing. And this problem started when I installed iOS 11.3 Beta 5, but now my iOS version is 11.3 release.

I've asked on Google Gmail Help Form about this and I thought it was not Google's problem, see my question: Google Groups

Need help, Thanks a lot!