primary and seconary WAN connection for IPSec tunnel

Hi guys,

I am new to the field of advanced routing. In my company we have the following network construct to a branch office:

• A dark fibre line connects 2 fortigate firewalls
• An LTE line is to be used as a backup line
• Both fortis are connected to each other via both lines using IPSec

At the moment the internet traffic goes over the LTE line, but in the future it should work as a backup internet line, but currently the LTE line is the internet access line for all clients in the branch office.
If we put a new default route 0.0.0.0/0 on the WAN interface with the dark fibre, both routes go down.

How do the two Fortigates have to be configured so that everything runs via the dark fibre and the LTE line is only used if the dark fibre fails?

Thank you in advance for your answers.