Thanks griff w but unfortunately found nothing of help there.
"keeps asking me to approve security code and when I write it, it asks me again. And keeps repeating. I chose approve from other device (I have a MBA) but nothing appeared on it. The Keychain button in iOS setting says pending."
Exactly the same here.
The AppleSupport article is useless it skims by saying, "follow instructions on screen"
Essentially if you are stuck in this pending loop. You're stuck !
What you'll probably have to do is reset your keychain. you can do that by going to (on your Mac) System Preferences > iCloud > select Keychain (and enter your password) > Use Code... > Forgot Code? > Reset Keychain...
Then you can go ahead and recreate your keychain from any device of your choosing (Mac, iPhone, iPad).
The device you choose will have it's keychain data uploaded to iCloud and be the master keychain that the rest of your devices will then merge with.
-hope that helps