After you append the ?kiosk=true to the URL, I presume you are pressing Enter to load the new URL and you are able to verify that the kiosk mode is working in the browser as expected before you try the Add to Homescreen approach?
Here’s the relevant help article for Kiosk Mode:
And here’s the community post for the Add to Homescreen approach:
A few things that come to mind that you might try:
- From the Dashboard list, tap the
...next to the dashboard you are interested in and use the View (Kiosk Mode) option and verify the nav bar is hidden as expected - Then pull-to-refresh the page so it reloads and then try using the Add to Homescreen approach.