WaniKani doesn’t have an app-for any device (at least not yet). The one you are using is user-created, and therefore, the WK team would be unable to help you.
I would directly contact the app manufacturer with questions and concerns, as not much can be done here on the forums.
Also, keep in mind that you can access the website on your device to do your reviews on your smartphone.
The app was rejected by Apple because it requires an external subscription that can not be purchased through Apple. So sadly it’s gone from the app store and unless @cplaverty has some secret magic up his sleeve, it will not and can not return or get updated.
It definitely sucks, but for the time being we’ll just have to use Safari (or Tsurukame, but it will inevitably hit the same wall eventually) until WK develops their own native applications that can tie into Apple’s subscription models.