ReaSet is already running full screen.
Safari on iPhone cannot hide its bars from a web page. Add ReaSet to the Home Screen instead — it launches with no address bar and no toolbar.
1Tap Share in Safari
2Choose Add to Home Screen
3Open ReaSet from the new icon
If the icon opens with an HTTPS error, turn off “Warn before connecting to a website over HTTP” in Settings › Apps › Safari. REAPER serves plain HTTP.