I have an old ipad2 with iOS 9.3.5 and I finally GOT IT TO WORK!
Came across this post, because I was struggling with the same issue. Authentication via the UI gives me a wrong username of password error.
SOLUTION:
There is an option within HA to bypass to login screen, explained here:
Note that the userId is not the username, but the ID. this can be retrieved via the users config screen (http://homeassistant.local:8123/config/users)
After making this change, you can access HA with safari (or any other Kiosk browser app) by clicking trusted networks option below the login/password box :