The latest iOS version is 12.1, i just updated mine to this version minutes ago. If you are running iOS 11, you are an older version since iOS 12 was release last september 17th.
I just tried logging in on my iOS device and found no issues logging in different ways. Is this your only device running True Key? If it is not and you have another trusted device running True key, please see if you can update your iOS to the latest and try to reinstall true Key on it, I suspect an issue on your client.