Here is the Captive Portal. Yo must reffer all .css and all .js from other server. In the code I have written "myhelpserver". You also have to permit http and/or https to this "myhelpserver" in the role noAuth to permit the access.
You have two zip files.
1.- CaptivePortal.zip it have the package to upload to the controller.
2.- myhelpserver.zip it have all the .css and .js to put in the external server.
Modify de html code as your risk ;D
Let me know if yo have any trouble to use it.
https://we.tl/3gyxul9ZrT
The link wil be up 7days.