VIPs with multiple WANs or external IPs

Hello all the community, I hope you can help me or guide me in what I intend to do or if it is possible to do it with my Fortigate 300E or if I need any additional software or hardware, my situation is the following:

Create a VIP to publish a web server name: "MY WEB SERVER" WAN interface 1 Type Static NAT External IP address / range 187.210.xx.xxx Mapped IP address / range 172.16.xx.xx

That way it works perfectly, all external requests are made through WAN 1, the problem that I have very frequently is that it falls constantly, I already have another ISP "WAN 2" what I intend is to add my WAN 2 to this same web server to that when my WAN 1 fails "MY WEB SERVER" stays online by WAN 2 until WAN 1 is re-established.

And if this is not possible with my fortigate, what options do I have to achieve more availability in external connections to my web server?

Thank you