Doing reviews is currently not possible in iPad. The Safari browser version of the page stopped working two months ago, and DuckDuckGo browser only loads a blank black page since yesterday.
I am paying the salary of the people responsible for this, but not for long.
Try using the tsurukame app. I’m on iPad and it’s all I use. It took a while to get used to it after only using native wanikani at first, but it has much better options for how to do set up preferences for lessons and reviews. It also has different font options, which is good practice for encountering kanji in the wild.