Beacause of space , I prefer wireless connection between router_1 and router_2.
Both run openwrt , I want they both use mwan3 for better network performance.
So how should I connect them?
It is not that easy, but it is doable.
You have router 1 with WAN1, and router 2 with WAN2.
First. Set router 1 as default gw in your local network.
The best way is to use vlans to make WAN traffic connection between routers.
On router 1 set its local WAN1 as default outgoint traffic, and make VLANxxx as secondary WANxxx, which is connected to router 2.
Over VLANxxx there will be sent all traffic to router 2 in case if WAN1 is down.
Now on router 2 set routing of traffic incomming from VLANxxx into its WAN2.
and that is all. it will work.
In case if WAN1 is up, all traffic from your computer will go as follow:
computer - router 1 - WAN1
and in case if WAN1 is down, traffic will go as follow:
computer - router 1 - WANxxx - router 2 - WAN2