Sophos Central Wireless with Captive Portal - random automatically logout


first sorry for my english.

We use Sophos Access Points APX120 managed in Sophos Central.
Here we created a SSID with a captive portal. So the user only must acceppt the terms & conditions to get in the internet.

This works fine.

After a random time of minutes, sometimes up to 5,6 hours the devices will automatically disconnect from the SSID.
Not all devices at the same time, but one after another during some minutes/hours.
I don't unterstand why. What's the general timout of a captive portal? 

What is wrong on my Configuration?

Thank you for help.

[edited by: newit1 at 12:52 PM (GMT -7) on 31 Aug 2022]