There's a known issue on iOS with external keyboards in a WebView. If you have C3 "added to homescreen" as recommended then previews appear in a popup WebView. For some reason iOS will not actually create keyboard events in the popup unless the user selects something that resembles a text box. If you use construct straight from Safari or use remote preview then the keyboard will work.
Alternatively there is also an unpleasant workaround that if you add a button to the layoutview, once it has been touched by the user the keyboard will work.
Games published to the web and run through iOS will not experience this bug.
As it's an external Apple bug we can't do much other than wait for them to fix it. We haven't been able to find a good workaround for it unfortunately.