Hi Jonas
You are correct.
If you manually open a browser and try to browse to a URL that starts with https:// (or a site that only uses https://) - then the AP cannot redirect you to the splash page, because it cannot intercept the traffic. Therefore, it will time out. To reach the splash page, you have to navigate to a http:// site. I understand this is difficult to have users understand and they will typically just try and search Google from their browser, which will also fail as Google use https:// also.
This isn't a Meraki thing, it's an industry standard problem with captive portals and https (SSL). If you try the same on another make of AP you'll see the same behaviour. It's just the way it is I'm afraid. Some AP manufactures try to intercept the https:// request, but this IMO is worse because it then throws a big SSL warning/error page to the user saying the Certificate is invalid any your session might be hijacked (man in the middle warning). this happens because the SSL certificate provided is not the real one, but one provided by the AP and of course the host name on the certificate does not match.
But, the issue here is that you shouldn't get to a position where you need to open a browser and visit a http site. You should automatically get the CNA popup which will then correctly show the splash page on the device.
P.S. There is no problem hosting your splash page on https:// - because the splash URL is in your allowed Walled garden list, so that traffic CAN reach the real site.
The CNA check on iOS/Android/Windows etc is always a http:// request by the way,otherwise that would fail.
Thanks
James
https://purple.ai