Many thanks to both for your suggestions. I have already tried both to no avail.
The Planet Keyboard app is not in Google Play. The app still appears in the Settings-Apps list, but the only options are to disable and force-close.
I had also previously tried the "restore to default", but it did not work. Strangely, going back to the menu again, I cannot toggle that option anymore.
Maybe when Linux comes around eventually, I will do a factory reset and hopefully see it back.
[Edit]: The app name in Cosmo is "Planet Keyboard". I also searched for "Gemini Keyboard" in Play Store which turns up, but it says it is already installed, and there is no "Open" button (there is no button at all). Disabling the "Planet Keyboard" app in the Cosmo causes the "Gemini Keyboard" to display the "Enable" button. So it is actually the same app, but it gets me no further.