Dual DHCP VLAN wan & LLB - default gateway

Hi there, my name is Miguel and I am from Spain. In my company we have recently bought a Fortigate 60E.

This is my configuration:

- 2 wan configured (VLAN type, DHCP configuration)

- WAN LLB with both wan interfaces (Type WAN Links Interface)

- Firmware version is 5.4.1

- Firewall is 60E

Everything is working fine.

The problem is:

when I configure LLB, I have to write down manually the wan gateway for each wan (I cannot leave it blank). As they are DHCP, it is working now, until the ISP change the gateway via DHCP. In that case, LLB won't update.

How can I configure LLB so it takes automatically the default gateway of each wan?

Thanks