If you are using a Cisco WLC must likely you have the "captive portal bypass" enabled. This is enabled via CLI with the command config network web-auth captive-bypass {enable | disable}.
If you enable it, the pop-up won't work. If you disable it, you will get the popup. However, is important to mention that the self-registration process might not work because the popup uses a pseudo-browser. I recommend that you enable it and test with a few devices to validate that it works. Also, keep in mind that changing the status if this feature requires a WLC reboot.
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