The default behavior in Freshdesk is that the users will be redirected to the Portal Home page when they access the Customer portal.
If you are looking to show the Tickets page or the Solutions page instead of the portal home, below is a small hack to do that.
Please add this code under Admin --> Channels --> Portals --> Customize portal --> Layout & Pages --> Portal pages --> Portal home.
<script type='text/javascript'>
window.location.href = 'https://domain.freshdesk.com/support/solutions';
</script>
This code will redirect the user to the solutions page when they access the portal home page. Similarly, You could replace the URL with the tickets page URL to redirect users to the tickets page.
This is available only from the Estate plan onwards.
Note: In the code, please replace domain.freshdesk.com with your Freshdesk URL.