I have this exact same issue. I do not have the advanced security license and therefor am not using content categories nor threat categories. I am doing all firewall and security settings at the "Wireless" level and not the "Security and SD-WAN" level. WPA2 only, 802.11r enabled, 802.11w enabled. Captive portal strength block all access....,  walled garden disabled, external dhcp server assignment (single vlan), no group policies used, DNS nameservers - proxy to upstream DNS (also tried google public dns).  Until I click on that windows side prompt "open browser and connect" the browser page and splash page will never trigger and appear; but once I click on that windows prompt, i get a new browser tab that opens with the www.msftconnecttest.com/redirect and then it successfully redirects to my splash page and then everything works perfectly fine. I have tested this also with a UniFi wireless network that uses a splash page and get the same windows side prompt about open and connect. This doesn't appear to be strictly limited to meraki splash page. ... View more