Daul-WAN failover using a Cudy R700 in front of your router

I've been trying to get dual-wan failover working reliably for a couple of months. First I tried to set it up directly on my router, an Asus RT-AX86U running Merlin. Sort of worked but there were issues, mostly related to the VLANs. Tried a different approach with an add-on software package, which again sort of worked but still had issues. I'd rather not replace the router; it performs well and is set up the way I want with a Pi-hole/dnscrypt-proxy and VLANs for guests and IoT, so I decided to try a separate box in front of it to do the WAN switching. The Cudy R700 at ~$40 seemed a likely candidate, with one WAN and 4 LAN/WAN ports. It's solidly made and presents a nice custom interface on top of OpenWRT. If your WANs are either decisively up or down, it would probably work well for you out of the box. Unfortunately, the failure mode of my primary WAN often isn't to fail completely, but to become unusable. The R700 stock software doesn't deal with that well. I almost returned it before finding that one could supposedly flash it with the full, current OpenWRT, which allows line quality to be taken into account when determining failover. So I gave it a try. And it worked!

Here's how to get full OpenWRT on the Cudy R700. [Note: there is always risk in flashing. Not responsible for bricked or nonreturnable devices.]

First flash the Cudy-signed image here: https://github.com/openwrt/openwrt/pull/18532#issuecomment-2818895606

Then go here to download the current OpenWRT image: https://firmware-selector.openwrt.org. Enter "R700" and choose the sysupgrade. Flash it.

Now follow the instructions here https://adityarajsingh.com/how-to-set-up-dual-wan-failover-on-openwrt-with-mwan3, with the following notes:

- Before step 2, run uci show network and note how the bridge is listed. It was different on my box vs the instructions. Adjust accordingly.

- Refer again to the uci show network output, which in my case showed 5 lan ports vs four physical ports, lan1 being an obvious system thing. The first LAN port I could turn into a WAN port was lan2, not lan1. Adjust if necessary.

- For step 5, OpenWRT now uses apk add instead of opkg install.

- Pay attention to the gotchas noted after step 5. I had to deal with 1 and 2. In addition, I needed to set wanb/lan2 to the WAN firewall zone before traffic was allowed from that port. Can be done in the web interface.

- Finally, I used the information here https://openwrt.org/docs/guide-user/network/wan/multiwan/mwan3-nft?s[]=start#interfaces_configuration to set line-quality parameters for failover (check_quality, *_latency, *_loss) and adjust the other parameters to my liking. This can be done through the web interface, or directly in /etc/config/mwan3.

The result is so far working well: quick failover to backup when the primary WAN degrades or dies, and a slower, more careful return.