Actually, apple devices try to reach out to captive.apple.com once you connect to a WiFi network, which automatically creates a HTTP GET request and therefore your splash page should kick-in. If this doesn't happen, it might be due to several reasons such as a firewall blocking access to that page or similar. I suggest you take a pcap and check what's happening when you client associates to a SSID.
Sameh Sackla - Cisco Meraki