Zywall USG 1100 captive portal not showing up on iOS devices

Bel

Hello!
Here is a tricky question. We configured a Web authentication on our Zywall 1100 so that we could authenticate clients from a specific network (Wi-Fi) using external Radius server. It works great on Android devices and when you connect to Wi-fi and try to open any page using http. It redirects correctly to authentication page. 

But there are 2 issues: 
When you try to open some website using https, it does not redirect to required service since there is a certificate mismatch.
But what more troublesome, the iOS devices does not show a captive portal at all. 
I also suspect, that this is a https related issue.

I contacted support, but they send me to Apple %)

Where should i dig? 

Zyxel_Charlie

Hello Bel,
For this case,
the mechanism of Web captive portal support http only, so please follow correct setting to avoid this situation occur.
Go to Configuration >System>WWW>Disable "Redirect HTTP to HTTPS" and the clients will be redirected to authentication page(Mobile) after connected with SSID. The PC user need to type ex: www.yahoo.com on browser, and then will be redirect to authentication page.

Secondly, I tested the IOS device(iphone 7, IOS version 10.2) locally, and it can be redirected without  any issue.
Charlie