The page didn’t appear on an iPhone
Usually because the phone remembers joining before. Forget the network.
iPhones cache the fact that they have already been signed in. Once that happens the sign-in window stops appearing, even though nothing is actually wrong.
Settings, then Wi-Fi. Tap the small i next to your network, then Forget This Network.
Turn Wi-Fi off for thirty seconds, then back on, and rejoin.
If it still does not appear, open Safari and go to neverssl.com.
Why neverssl.com? Almost every site a phone loads is encrypted, and encrypted pages cannot be redirected to a sign-in page. That site exists specifically to be unencrypted, so it forces the portal to appear. It is the standard test across the whole industry.
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.