There is no single, foolproof method to have the captive portal page show up when you connect through the router.
Turning off DNS Rebinding Attack Protection is the first thing to try. Next is to try opening the page http://neverssl.com or http://1.1.1.1 (http, not https). Another method is to connect directly with your phone/tablet/computer then cloning its MAC address onto the router.
Recently, I was in a hotel and what worked for me was just to change the DNS setting on my tablet to 1.1.1.1. By default, the DNS is the router’s LAN IP address.
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