I could be wrong but I’m rather sure I read a post somewhere in this forum the ‘Captive Portal’ in this context is if your Beryl itself was to be a ‘hotspot’ as if it was a coffee-shop Wi-Fi AP.
I recall reading that feature was removed by GL because it didn’t make sense to have a primarily business oriented feature on a consumer travel router.
You can always make a backup of your settings, try it, reflash to a previous f/w version if you don’t like it: