Mwan3 for load balancing between two 4G routers with internal modems

I have two routers with internal 4G modems configured with LEDE.
I want to load balance between them using mwan3 on one of the routers.

I have followed the mwan3 documentation here:-
openwrt mwan3 how to
and I have created a second WAN on the first router on a spare port on the switch and set it up with a static IP address within the same range as both routers. The second router is connected to this second wan port on the first router. The mwan3 load balancing works but both WAN ports on the first router are currently in the same firewall zone (as a result of following the mwan3 documentation where it states “assign the new wan2 interface to the wan firewall zone”) so the connection to the second router is natted.

What would be the correct procedure to follow to allow access to the second router via the second WAN port on the first router without natting and retain the load balancing functionality?

Hi? Did you solve your problem ? how?
I am interested on the 4G Modem/router, which device is it?

Sorry about the late reply.
I use a TP-Link Archer MR200.
I never progressed much with mwan3 though.