The page didn’t appear on an Android
Check the notification shade first, then use neverssl.com.
Android usually posts a notification rather than opening a window, and it is easy to miss.
1
Join the Wi-Fi, then swipe down from the top of the screen.
2
Tap the Sign in to network notification if it is there.
3
If not, open Chrome and go to neverssl.com, which will force the page to appear.
Some Android phones also need "Switch to mobile data automatically" turned off. Otherwise they drop back to 4G once they decide the Wi-Fi has no internet, which is exactly what a sign-in page looks like to them.
More in guests can’t see the page
If this did not solve it
Email hello@margovy.com. Tell us the venue name and what you were doing when it went wrong, and a person will reply.