In step 2 what do you mean by ssl plugin? Some plugin to wordpress? Also, where these other steps are configured?
Steps to add SSL support on portal area page only:
1. Install & Configure WHMCS Bridge : https for WHMCS URLs in both wordpress and WHMCS
2. Install HTTPS plugin
3. Add SSL Host Domain
4. Check Force SSL Administration * optional*
5. Check Force SSL Exclusively
6. Leave Remove Unsecure Elements uncheck it breaks the portal.
7. Add /portal-page/((?:[a-z][a-z0-9_]*))/ To URL filters this prevents Force SSL Exclusively feature from changing all the form urls to non https.
Note: / portal-page / needs to be changed to your permalink url.
8. Go to portal-page and click the Secure post option and save