I've had this issue for awhile, if you go to Settings, Wifi you will get a refreshing symbol and then the splash will come up sometimes I used to have to shut off/on wifi but I haven't had to do that in awhile. It happens on Androids as well but far less often.
when you first connect to guest Wi-Fi , If it is Android or Windows PC, The splash page pop up automatically without go to any website, but Apple device doesn't . I expect splash page can work same on Apple device
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.