Might want to provide a link to the guide you followed.
From the Meraki point of view, this is the guide to look at to configure EXCAP (External Captive Portal):
https://documentation.meraki.com/MX/Access_Control_and_Splash_Page/Configuring_a_Custom-Hosted_Splas...
From your screenshot I'm wondering whether you performed this step:
Enabling a Custom-Hosted Splash page on the Meraki Cloud
- Navigate to Configure > Splash page
- Select the SSID you want to configure from the SSID drop-down.
- Under Custom splash URL select the radio button Or provide a URL where users will be redirected:
- Type the URL of your custom splash page:
- Click Save Changes.
Purple will need to be compatible for this to work properly!
Rakesh, here's the OP's screenshot more readable:
Link