Hello there!
I am in a unique situation here.
I have an Asus router (on which I haven't installed Openwrt yet, since it is still in warranty). This is connected to the internel using an optical modem. I have an ethernet backhaul connected to another router running openwrt and acting as a dumb AP. Lately I have been having issues with my ISP, with the network randomly going down. So I want to add a second internet connection.
Now the thing is, due to the unique situation of wiring in my house, I can put the second internet connection to the 2nd router only (the one running openwrt).
Is there any way to configure the network in such a way that 2 routers each have it's own WAN, only one router handles DHCP with all devices on the same LAN, but if any one WAN fails then the devices connected to both routers could access the internet over the working WAN from any router?
Yes, this is possible, however neither router is going to support the configuration you need for doing so, because they are all built to run on their own. This is also beyond the scope of OpenWrt.
Multiple WAN routers with fail-over DHCP is a task for Pro equipment running something like Cisco IOS installed by a network engineer.
DHCP would not need failover since I am not talking about a high availability situation to protect against the router going down. I only want WAN failover, with the only unique thing being that the WAN is on 2 different routers.
If both are running OpenWrt you can speak vrrp between those routers but you will need to write some scripts which get triggered by keepalived which would i.e. edit and restart dnsmasq which handles ipv6 RA.
It is not dead simple but also no rocket science and do able .
I could install openwrt on the Asus router, it seems it is supported. I need to look up on vrrp, it seems that was the term i was looking for.
But in a very crude (proof of concept) way, I could health check the primary router. Have a virtual ip shared between both routers as the gateway, and whenever network access fails on one, move the virtualip to the other.
Does this make sense?