Why doesn't an iPhone with no cellular data open WiFi captive portal

Recently had some family visiting from overseas. Some didn’t have roaming data, and it seemed that their phones would sometimes just never open captive portal pages to log in when connecting to free wifi places. Now I’m having same problem after my carrier blocked my data because my payment card expired. Even when I’m connected to wifi, and expecting a pop up from the os, or a browser redirect if I go to http page, to the Wi-Fi login pages, nothing loads. Does iOS have to try making a connection on the wwan before trying wan, and the wwan is sending my packets into a ‘no packet gateway bearer for you’ black hole, in a way that doesn’t time out, so iOS never tries wan to get the redirect? Anyone know what’s happening and how I could work around it. Authenticated Wi-Fi works fine.