Radius User attributes (session control)

Hi, I'm trying to develope a custom captive portal. All is working but i'm unable to figure out how it works the fortigate and the radius attributes related to the session time control. The only attribute that works for me is the session-timeout but seems to have a limitation I can't use high values because if I use a value of 5000 seconds for example the user is configured with the default value 300. I would use the expiry value but if I use that attribute the user is created again with a 300 seconds timeout.

Thanks.