Are you using wifi or your iPhone cellular service?
Are you connecting with HTTP or HTTPS?
Which IOS release?
Can you please contact Esri Tech Support and log an issue? We've had a couple of other similar reports, but we can't seem to get a definitive repro case to investigate further. \
Do you know if this is a known issue with iPhone? Because my https GeoForm works fine in Android devices, but not on any iPhone devices. Here is the GeoForm: https://arcg.is/1zPTzq
I believe this was recently noted as a known limitation with IOS.
Please see this GeoNet post by agup-esristaff for more details,
Hope this helps,