Fallback with non-bridged 4g router to fw

Hello,

I have a 4G router with a static public IP address that I want to use as a fallback to my fiber router in bridge mode using a Fortigate firewall. This 4G router only as one ethernet port.

Right now I have a IPv4 policy route from internal to wan1 (internal uses IP range 10.20.110.0/24) and a static route forwarding all traffic to wan1 (pppoe, router in bridge).

My plan is to create another static route routing all traffic to the 4G router (wan2) with the same administrative distance as the other static route to wan1 but with lower priority so wan1 keeps being the main one until it fails and jumps to wan2.

But, since the router connected to wan2 cannot be set in bridge mode it has its own internal network (192.168.8.0/24), dhcp, dns... etc. so I don't know how am I supposed to route the traffic from internal to wan2 if internal will have a different internal network than the router's (wan2) internal network. I also don't know how to set up wan2 as an interface (I know I have to set it as manual but I'm not sure about filling the rest of the fields). Should I connect 10.20.110.0/24 to the wan1 and NAT as it if was coming from, for example 192.168.8.100/24? I'm kind of lost.

Any help is welcome, thank you.