OK, I just tested my fail-over VPN and it appears to work on my system.You'll need to create two VPN instances, one for the LTE and one for your main wan. Now create a gateway group for those two interfaces. Remember you'll also need to setup and enable gateway monitoring for both VPN gateways, use anything you like, google dns whatever, but they must all be different, this includes your normal gateways.I have a something extra, I only want to route a couple of PC's through the VPNs, so I have an alias setup with the addresses that I want to use the VPN, It's just called ExpressVPN hosts and contains the IP Addresses of the LAN clients that will connect to the internet via the vpn. Under the LAN firewall rules I have a rule for my ExpressVPN Hosts that says their gateway will be the gateway group I created for the VPNs.Finally, under NAT, I am using Hybrid outbound and I have rules for both VPN interfaces with the source being my ExpressVPN hosts and the NAT address of each VPN Interface.And it works. I can unplug either NIC and it fails-over to the VPN on the still connected NIC. Whether it work for the way you have your VPNs setup I don't know, but it works when using something like ExpressVPN where I run the two clients.