Access point with captive portal (Hotspot)

Dear All,

We are running some tests with APX and AP6 access points managed on Sophos Central.
Our APX access points are all declared on our XGS3100 and working fine with 3 types of SSIDs.
One is a bridge mode to our LAN, one is for or mobile devices and a third one is for Guest users with a captive portal and Voucher access.

We were able to declare the first two modes (bridge with MAC address filtering; SSI for mobile devices on a dedicated VLAN)
Now we are trying to make the Captive Portal work, with Voucher security.
When I try to connect a laptop to the SSID with this mode, I never have the captive portal.
I had it once, and now it simply stop working.

How can we troubleshoot what is happening ?

Best regards,
Octavio