Apple Phones are disconnected after screen is locked and have to re-authenticate

Good day Folk, We currently used Fortigate 100D with Captive Portal configured on our Staff network, the Access Point devices are Ubiquity and Mikrotik; users having Apple Phones are been disconnected after they locked the screen of they are phones and have to re-authenticate again (Very painful); Android, PCs and Laptops are not affected. The current version of the firmware is 6.0.4 Below are a part of my config: config system global     set alias "FG100D5Dxxxxxxxx"   [style="background-color: #ff0000;"]  set auth-keepalive enable[/style]     set block-session-timer 300     set compliance-check disable     set gui-device-latitude "-11.23715097876325"     set gui-device-longitude "10.26335970343895"     set gui-lines-per-page 200     set gui-theme red     set hostname "TooCool_FW"     set policy-auth-concurrent 5     set proxy-auth-timeout 5     set refresh 30     set switch-controller enable     set timezone 34 end config user setting     set auth-type http ftp telnet     set auth-cert "Fortinet_Factory"     [style="background-color: #ff0000;"]set auth-timeout 1440[/style]    [style="background-color: #ff0000;"] set auth-portal-timeout 30[/style] end I have enabled auth-keepalive and increased auth-timeout time and removed auth-secure-http in user setting; but still having the same issue. Please can any one assist me? this issue has been happening for a very longtime now. Thank you