When users use the iOS mobile client of an earlier version to log in to the WI of a later version, "Please install the latest iOS client" is displayed after users click the login icon, and users fail to log in to the WI.
Download the iOS mobile client of the latest version from the WI or App Store, and use this client to access the WI.
If you cannot download the App, try to clear the cache. Using the iPad as an example, tap Settings > Safari. In the list on the right, tap Clear Cookies and Data.
The version of the iOS mobile client is an earlier one, and the client must be upgraded to the latest version. Another possible cause is that the information about the WI of an earlier version is stored in the session of the iOS mobile client.