Seems to be the same here:
- FGT-800C, FW 5.2.4
- Using a captive portal on an open SSID
- Using Guest accounts
iDevices keep disconnecting after a few minutes, even when the used is actively browsing the internet.
I tried to bypass the CNA by allowing FQDNs coming from iDevices, which works BTW. The device when it connects to my open SSID does not pop the FGT login page. It does pop the first time I use Safari though. I thought that maybe would solve the problem by allowing the connection refresh page to be kept, however, I get the same disconnects.
My iDevices connected to another SSID that uses RADIUS to authenticate does not exhibit the same behaviour.
I saw nothing in the release notes for 5.2.5, which supports throsoul's observations, so I'm not hopeful an update will solve the problem.
Fortinet support can reach me for information if needed. It would be a huge help to fix this problem.