Failover IPSec using SDWAN & VDOMs

Hi,

I have FortiGate-100E & FortiExtender setup locally in SDWAN. Similarly, FortiGate 30E & FortiExtender on remote site in SDWAN as well.

VDOM-A in FortiGate-100E connects to Internet through root VDOM. There's IPSec VPN between VDOM-A and remote site (DSL) using inter-vdom links, physical interfaces and VIPs. This works.

Now, I'd like to setup failover by creating additional tunnel between VDOM-A and 4G router in the remote site. I've created vpn tunnel in the remote site using pretty much same configuration except the interface is set to FortiExtender. This tunnel is not coming up.

Also, I'm using DDNS for both gateways. Any ideas, solutions, hints would be highly appreciated.