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.
Have you tried other browsers? Because it sure works for me. You can also, always contact the WK team directly for assistance. 
The timing does suggest things breaking for you during the big update? 
The email is hello@wanikani.com
I only use iPad and it works fine for me
Works for me on iPad with both Safari and Chrome.
Working fine on the latest iPad Pro
You should stop paying Apple
Silly, part of your paycheck goes to Apple regardless of what you buy, or don’t
I don’t get paycheck. I get stock options.
Finally normal people
The penguin wearing a top hat said to the Nic Cage vampire
And both from Berlin. Perfect.
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.