Redundant Internet without Load-Balancing or Link Aggregration

Hi all,

Right now we have HA Fortigate connected to 2 ISPs. We use ISP1 for servers' connection and ISP2 for users (web browsing, email and stuff), so we don't bundle the interfaces for redundancy. We have 2 static routes for 2 ISPs and 2 policies for each ISP to connect to the internet, but in case of ISP2 goes down, we want ISP1 to handle the internet connection for users and vice versa. I read the documentation but couldn't find how to achieve this.

Any guidance will be greatly appreciated.

Regards.