Issues with hotspot networks on Apple devices
Apple devices are known to have several issues with captive portal hotspots. While specific behaviour will depend on a particular device model, OS version and possibly other factors such as geographic location, and as such might or might not present themselves, there are a few workarounds that might do the trick for your device.
Apple devices disconnect from hotspot
Thread on Apple’s official forums
Another thread on Apple’s official developers’ forums
Link to a GitHub thread regarding Apple devices disconnecting after locking the screen
Apple devices not displaying captive portal after connection
Thread on Apple’s official forums
Another thread on Apple’s official forums
Yet another thread on Apple’s official forums
Apple official troubleshooting guide
Captive portal networks working slowly on Apple devices or not loading
Thread on Apple’s developers’ forums
Thread on Reddit regarding issues with Apple captive portals
Thread on Ubiquiti forums regarding issues with loading a captive portal