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Dual WAN & bridging modem

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My ISP provides me with a modem with a regular internet line and a backup, which is a 4G LTE USB modem plugged in to it. Today, during an internet outage, it appeared that the 4G backup wasn't working as my AX86U didn't receive an IP anymore.

After discussing this with the ISP, it turns out that in normal operation, the modem is only bridging, so my Asus MAC is registered with them and gets a public IP directly. When the main line fails, the 4G modem takes over the WAN connection of the modem, but bridging breaks. So, only devices connected directly to the modem (/its internal router) keep working as they still get a private IP.

Now, I was wondering if I could use the dual-WAN feature to overcome this. 2 Items come to mind:

* When I connect a cable between Asus-LAN and Modem-LAN and set it as WAN failover, the ASUS should have a secondary uplink that can use the 4G link. Will this work seemlessly if that Asus interface gets a private IP instead of a public one?
* If I connect that cable, will the modem see the same MAC as it gets from the Asus-WAN port or will this be a different one? In the former case, the approach won't work.

From here it's not easy to understand how the ISP modem manages the 4G backup connection and how it could be possibly externally controlled by your Asus.
If the ISP modem has internally all the logic for establishing a backup connection in case of main line dropout, maybe you could simply set the ISP modem to work as a router and setup on that modem a DMZ for your Asus.
If you don't need a public IP on your Asus, IMHO this is the simplest way.

Maybe another possibility could be to connect the 4G USB dongle directly to your Asus and let it manage the 2 WAN connections.
According to the ISP, the dongle won't work on the router (although I guess I could test it). As for DMZ, this is supposed to be supported, but I've seen people complain that the DMZ function is being deprecated by the ISP so this may not be a long term fix. Also, I'm running quite a few services on the Asus which I'm not sure will work behind another NAT... :(
I'm using 3 WANs, 2 with provider supplied modems that are required to create the PPPoE connections so to get the assigned Internet static IPs, and 1 failover 4G WAN using a 4G modem which as known doesn't support a PPPoE connection and even gets only private IP addresses from the carrier.

So all the WANs are set with DMZ to my VPN firewall and everything works (dozen of services http e https with port redirections to the internal LAN).
In my case (and in similar situations) DMZ is the only way, no other possibility, and also NAT overhead is neglectable, less than a 1 ms. delay.

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