Captive portal behind a NAT

Hello guys , hope you all doing great !   I want users from the internet, when typing my website's public address , to be redirected to fortigate's captive portal, so they can provide their usernames and passwords before gaining access to the website.   My fortigate unit is behind a router;   The Fortigate's interface connected to the router is NAT translated in the router with the public address and port 1000 ( the port used to redirect users to the captive portal );   The website is also NAT translated in the router and all policies allowing trafic from the outside to the website and the captive portal's interface are configured;   BUT the redirection is not working from internet ( timeout problem ) . I have done the test localy and it's working great.   Hope you guys have some tips to help me.    Thank you.