When using Captive Portal for User Authentication (i.e. Wi-Fi User Authentication, Local User Authentication without SSO, etc.), the page may not load correctly on iOS/Android devices hence it's not possible to login.
This issue is due to popup blocker installed on mobile operating systems.
To resolve this, you can enable the following option:
Go to Users | Settings
Under User Session Settings for Web Login, enable the option "Open user's login status window in the same window rather than in a popup"