backup port in fortigate

Obviously I don't know every detail of your network, but take a look at this:

https://community.fortinet.com/t5/FortiGate/Technical-Tip-Creating-a-redundant-link/ta-p/196577

It might be what your are looking for.

Best regards.

You can go with aggregating the ports (LACP) to achieve redundancy.

Regards,

You have two options:  Aggregation and redundancy

Link aggregation (IEEE 802.3ad) enables you to bind two or more physical interfaces together to form an aggregated (combined) link. This new link has the bandwidth of all the links combined. If a link in the group fails, traffic is transferred automatically to the remaining interfaces. The only noticeable effect is reduced bandwidth.

In a redundant interface, traffic only goes over one interface at any time. This differs from an aggregated interface where traffic goes over all interfaces for increased bandwidth. This difference means redundant interfaces can have more robust configurations with fewer possible points of failure. This is important in a fully-meshed HA configuration.

Reference:

https://docs.fortinet.com/document/fortigate/7.4.0/administration-guide/567758/aggregation-and-redundancy