icloudtwo-factor-authentication
It would seem that the point of iCloud.com and its web apps is to provide access to your content on the go, when for whatever reason, you don’t have access to your device itself.
But if logging into the website requires you to first authenticate yourself with the device, then what is the point of the website?
Of course if you have your device handy — then you wouldn’t need the website…
How is this not a glaring oversight by Apple?
You can restore your device by putting it in Recovery Mode first. This will restore your device regardless of settings on the device, including Find my iPhone.
Hold Home as you connect the USB cable until you see the following screen:
You should see the following message in iTunes:
iTunes has detected an iPhone in recovery mode. You must restore this iPhone before it can be used with iTunes.
Restore the device in iTunes.
How to manually view saved passwords and credit card numbers with iCloud Keychain
Source: How to view your saved passwords and credit card numbers with iCloud Keychain
Best Answer
The first time you access a 2FA-protected iCloud account on any device you've chosen to use with 2FA, you must provide the correct credentials with that device to assure iCloud that it is indeed the device you've selected to receive 2FA authentication. You are then offered the choice of ignoring future requests for authentication on that device for some duration of time. Every so often, you must re-authenticate with the 2FA credentials to reassert your identity. Does this help?