iPad doesn't display the captive portal login window after connecting to a WiFi network
iOS attempts to connect to http://captive.apple.com. If this request doesn't return the string "Success", the captive portal page is opened on the device. Therefore if the router isn't redirecting this request, the captive portal won't be shown.

Usually in these cases you can open Safari and connect to a non-HTTPS URL where you will be redirected to the captive portal. If you know the URL of the portal through another device, you can enter this in Safari to go straight there if the redirects aren't working properly.

If DNS is really not working, you might even need to browse something like http://1.1.1.1 if you don’t see the captive portal log in. 

In the majority of cases (maybe 98%) this is a problem on the network,  it the iOS device. If you reboot your device and still can’t connect, you’ll need the person(s) that run the network to assist in all likelihood.