solution for 2 dynamic WAN with single DDNS for VPN

Hi,

im using FG80e with firmware 5.6

i have;

2 dynamic wan running 4g, using fortiddns. => branch

1 fix public ip => HQ

branch VPN to HQ with ddns with 1 dynamic ip WAN using DDNS no problem.

my questions;

branch

1) can i configure monitor 2 WAN interface at the DDNS there.

2) can my VPN still work? how to configure it or any cookbook?

3) this is a active-backup VPN

attached is simple diagram